4. How do you explain, and is their some significance to, the overlap between Bishops and Apostles both being present in the NT church itself?
Regarding this, there is an important point regarding those bishops not Apostles at the Council of Jerusalem, referred to as presbyters, regarding the derived word πρεσβεύομεν πρεσβεύω from the base of presbuteros; to be a senior, i.e. (by implication) act as a representative (figuratively, preacher) -- be an ambassador, i.e. akin to "apostolos/sliH".
2 Corinthians 5:20 ὑπὲρ Χριστοῦ οὖν πρεσβεύομεν ὡς τοῦ θεοῦ παρακαλοῦντος δι' ἡμῶν• δεόμεθα ὑπὲρ Χριστοῦ, καταλλάγητε τῷ θεῷ.
We are therefore ambassadors on behalf of Christ, as though God were entreating by us: we beg you on behalf of Christ, be reconciled to God
Ephesians 6:20 ὑπὲρ οὗ πρεσβεύω ἐν ἁλύσει, ἵνα ἐν αὐτῷ παρρησιάσωμαι ὡς δεῖ με λαλῆσαι.
for which I am an ambassador in chains; that in it I may speak boldly, as I ought to speak.
Christ is the apostle sent by the Father, the disciples are the apostles sent by Christ, the bishops are the apostles sent by the disciples. I don't think it has ever been put better than by St. Clement: “This man, as he had seen the blessed apostles, and had been conversant with them, might be said to have the preaching of the apostles still echoing [in his ears], and their traditions before his eyes. Nor was he alone [in this], for there were many still remaining who had received instructions from the apostles. In the time of this Clement, no small dissension having occurred among the brethren at Corinth, the Church in Rome despatched a most powerful letter to the Corinthians, exhorting them to peace, renewing their faith, and declaring the tradition which it had lately received from the apostles, proclaiming the one God, omnipotent, the Maker of heaven and earth, the Creator of man, who brought on the deluge, and called Abraham, who led the people from the land of Egypt, spake with Moses, set forth the law, sent the prophets, and who has prepared fire for the devil and his angels. From this document, whosoever chooses to do so, may learn that He, the Father of our Lord Jesus Christ, was preached by the Churches, and may also understand the apostolical tradition of the Church, since this Epistle is of older date than these men who are now propagating falsehood” (St. Iranaeus, “The Apostolic Tradition”).http://www.ccel.org/ccel/schaff/anf01.ix.iv.iv.html
"There is extant also another epistle written by Dionysius to the Romans, and addressed to Soter, who was bishop at that time. We cannot do better than to subjoin some passages from this epistle…In this same epistle he makes mention also of Clement's epistle to the Corinthians, showing that it had been the custom from the beginning to read it in the church....' Dionysius of Corinth, To Pope Soter" (A.D. 171).
St. Clement knew the Apostles, was appointed and ordained by them personally, and whose letter was read like Scripture for many centuries, yet did he ever claim the title "Apostle?" In I Clement he writes (c. 95, i.e. while the last Apostle John still lived):
...42 The apostles have preached the Gospel to us from the command of the Lord Jesus Christ; Jesus Christ [has done so] from God. Christ therefore was sent forth by God, and the apostles by Christ. Both these appointments, then, were made in an orderly way, according to the will of God. Having therefore received their orders, and being fully assured by the resurrection of our Lord Jesus Christ, and established in the word of God, with full assurance of the Holy Ghost, they went forth proclaiming that the kingdom of God was at hand. And thus preaching through countries and cities, they appointed the first-fruits [of their labours], having first proved them by the Spirit, to be bishops and deacons of those who should afterwards believe. Nor was this any new thing, since indeed many ages before it was written concerning bishops and deacons. For thus saith the Scripture in a certain place, “I will appoint their bishops in righteousness, and their deacons in faith."...44 Our apostles also knew, through our Lord Jesus Christ, and there would be strife on account of the office of the episcopate. For this reason, therefore, inasmuch as they had obtained a perfect fore-knowledge of this, they appointed those [ministers] already mentioned, and afterwards gave instructions, that when these should fall asleep, other approved men should succeed them in their ministry. We are of opinion, therefore, that those appointed by them, or afterwards by other eminent men, with the consent of the whole Church, and who have blamelessly served the flock of Christ in a humble, peaceable, and disinterested spirit, and have for a long time possessed the good opinion of all, cannot be justly dismissed from the ministry. For our sin will not be small, if we eject from the episcopate those who have blamelessly and holily fulfilled its duties and presented the offerings. Blessed are those presbyters who, having finished their course before now, have obtained a fruitful and perfect departure [from this world]; for they have no fear lest any one deprive them of the place now appointed them. But we see that ye have removed some men of excellent behaviour from the ministry, which they fulfilled blamelessly and with honor.
Another man, personally chosen and ordained by the Apostles, St. Ignatius, Patriarch of Antioch by the hand of St. Peter in his original see where "the believers were first called Christians," wrote (c. 105):
It is therefore befitting that you should in every way glorify Jesus Christ, who has glorified you, that by a unanimous obedience you may be perfectly joined together in the same mind, and in the same judgment, and may all speak the same thing concerning the same thing, (1 Corinthians 1:10) and that, being subject to the bishop and the presbytery, you may in all respects be sanctified…Wherefore it is fitting that you should run together in accordance with the will of your bishop, which thing also you do. For your justly renowned presbytery, worthy of God, is fitted as exactly to the bishop as the strings are to the harp.
(Ignatius to the Ephesians 2, 4)
Let no man deceive himself: if any one be not within the altar, he is deprived of the bread of God. For if the prayer of one or two possesses Matthew 18:19 such power, how much more that of the bishop and the whole Church! He, therefore, that does not assemble with the Church, has even by this manifested his pride, and condemned himself. For it is written, God resists the proud. Let us be careful, then, not to set ourselves in opposition to the bishop, in order that we may be subject to God….Especially [will I do this ] if the Lord make known to me that you come together man by man in common through grace, individually, in one faith, and in Jesus Christ, who was of the seed of David according to the flesh, being both the Son of man and the Son of God, so that you obey the bishop and the presbytery with an undivided mind, breaking one and the same bread, which is the medicine of immortality, and the antidote to prevent us from dying, but [which causes] that we should live for ever in Jesus Christ.
Ephesians 5, 20
Since therefore I have, in the persons before mentioned, beheld the whole multitude of you in faith and love, I exhort you to study to do all things with a divine harmony, while your bishop presides in the place of God, and your presbyters in the place of the assembly of the apostles, along with your deacons, who are most dear to me, and are entrusted with the ministry of Jesus Christ, who was with the Father before the beginning of time, and in the end was revealed. Do all then, imitating the same divine conduct, pay respect to one another, and let no one look upon his neighbour after the flesh, but continually love each other in Jesus Christ. Let nothing exist among you that may divide you; but be united with your bishop, and those that preside over you, as a type and evidence of your immortality…As therefore the Lord did nothing without the Father, being united to Him, neither by Himself nor by the apostles, so neither do anything without the bishop and presbyters. Neither endeavour that anything appear reasonable and proper to yourselves apart; but being come together into the same place, let there be one prayer, one supplication, one mind, one hope, in love and in joy undefiled. There is one Jesus Christ, than whom nothing is more excellent. Therefore run together as into one temple of God, as to one altar, as to one Jesus Christ, who came forth from one Father, and is with and has gone to one…Study, therefore, to be established in the doctrines of the Lord and the apostles, that so all things, whatsoever you do, may prosper both in the flesh and spirit; in faith and love; in the Son, and in the Father, and in the Spirit; in the beginning and in the end; with your most admirable bishop, and the well-compacted spiritual crown of your presbytery, and the deacons who are according to God. Be subject to the bishop, and to one another, as Jesus Christ to the Father, according to the flesh, and the apostles to Christ, and to the Father, and to the Spirit; that so there may be a union both fleshly and spiritual.
Magnesians 6-7, 13.
For, since you are subject to the bishop as to Jesus Christ, you appear to me to live not after the manner of men, but according to Jesus Christ, who died for us, in order, by believing in His death, you may escape from death. It is therefore necessary that, as you indeed do, so without the bishop you should do nothing, but should also be subject to the presbytery, as to the apostle of Jesus Christ, who is our hope, in whom, if we live, we shall [at last] be found. It is fitting also that the deacons, as being [the ministers] of the mysteries of Jesus Christ, should in every respect be pleasing to all. For they are not ministers of meat and drink, but servants of the Church of God…In like manner, let all reverence the deacons as an appointment of Jesus Christ, and the bishop as Jesus Christ, who is the Son of the Father, and the presbyters as the sanhedrim of God, and assembly of the apostles. Apart from these, there is no Church….But shall I, when permitted to write on this point, reach such a height of self-esteem, that though being a condemned man, I should issue commands to you as if I were an apostle? I have great knowledge in God, but I restrain myself, lest, I should perish through boasting. For now it is needful for me to be the more fearful; and not give heed to those that puff me up…And this will be the case with you if you are not puffed up, and continue in intimate union with Jesus Christ our God, and the bishop, and the enactments of the apostles. He that is within the altar is pure, but he that is without is not pure; that is, he who does anything apart from the bishop, and presbytery, and deacons, such a man is not pure in his conscience…Continue in harmony among yourselves, and in prayer with one another; for it becomes every one of you, and especially the presbyters, to refresh the bishop, to the honour of the Father, of Jesus Christ, and of the apostles…well in Jesus Christ, while you continue subject to the bishop, as to the command [of God], and in like manner to the presbytery
Trallians 2-4, 7, 12
I salute in the blood of Jesus Christ, who is our eternal and enduring joy, especially if [men] are in unity with the bishop, the presbyters, and the deacons, who have been appointed according to the mind of Jesus Christ, whom He has established in security, after His own will, and by His Holy Spirit. Which bishop, I know, obtained the ministry which pertains to the common [weal], not of himself, neither by men, Galatians 1:1 nor through vainglory, but by the love of God the Father, and the Lord Jesus Christ…I flee to the Gospel as to the flesh of Jesus, and to the apostles as to the presbytery of the Church. And let us also love the prophets, because they too have proclaimed the Gospel, and placed their hope in Him, and waited for Him…See that you all follow the bishop, even as Jesus Christ does the Father, and the presbytery as you would the apostles; and reverence the deacons, as being the institution of God. .
Philadelphians 1, 5, 8
Let no man do anything connected with the Church without the bishop. Let that be deemed a proper Eucharist, which is [administered] either by the bishop, or by one to whom he has entrusted it. Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. It is not lawful without the bishop either to baptize or to celebrate a love-feast; but whatsoever he shall approve of, that is also pleasing to God, so that everything that is done may be secure and valid. Smyrneans 8.
But shall I, when permitted to write on this point, reach such a height of self-esteem, that though being a condemned man, I should issue commands to you as if I were an apostle?
Pray, then, do not seek to confer any greater favour upon me than that I be sacrificed to God while the altar is still prepared; that, being gathered together in love, you may sing praise to the Father, through Christ Jesus, that God has deemed me, the bishop of Syria, worthy to be sent for from the east unto the west. It is good to set from the world unto God, that I may rise again to Him….Entreat Christ for me, that by these instruments I may be found a sacrifice [to God]. I do not, as Peter and Paul, issue commandments unto you. They were apostles; I am but a condemned man: they were free, while I am, even until now, a servant. But when I suffer, I shall be the freed-man of Jesus, and shall rise again emancipated in Him…From Syria even unto Rome I fight with beasts, both by land and sea, both by night and day, being bound to ten leopards, I mean a band of soldiers, who, even when they receive benefits, show themselves all the worse. But I am the more instructed by their injuries [to act as a disciple of Christ]; yet am I not thereby justified. (1 Corinthians 4:4)
Cf. [4 For I am not conscious to myself of anything. Yet am I not hereby justified: but he that judgeth me is the Lord. 5 Therefore, judge not before the time: until the Lord come, who both will bring to light the hidden things of darkness and will make manifest the counsels of the hearts. And then shall every man have praise from God. 6 But these things, brethren, I have in a figure transferred to myself and to Apollo, for your sakes: that in us you may learn that one be not puffed up against the other for another, above that which is written. 7 For who distinguisheth thee? Or what hast thou that thou hast not received, and if thou hast received, why dost thou glory, as if thou hadst not received it?
8 You are now full: you are now become rich: you reign without us; and I would to God you did reign, that we also might reign with you. 9 For I think that God hath set forth us apostles, the last, as it were men appointed to death. We are made a spectacle to the world and to angels and to men.]
And he concludes:
Remember in your prayers the Church in Syria, which now has God for its shepherd, instead of me. Jesus Christ alone will oversee it, and your love [will also regard it]. But as for me, I am ashamed to be counted one of them; for indeed I am not worthy, as being the very last of them, and one born out of due time. (1 Corinthians 15:8-9) Romans 2, 4-5, 9.