Let's agree on a methodology. Let's discuss Nestorius separately from Nestorianism as the two are not exactly co-terminous (the Assyrian Church of the East confesses Nestorius as a confessor, not as a doctrinal teacher in their tradition.) I will address Nestorius' teachings first, which clearly show he does not believe in the same type of inseparable unity as Orthodox (Eastern and Oriental) do.
Nestorius' 12 Anathemas to Cyril
1. If anyone says that the Emmanuel is true God, and not rather God with us,
that is, that he has united himself to a like nature with ours, which he assumed
from the Virgin Mary, and dwelt in it; and if anyone calls Mary the mother of
God the Word, and not rather mother of him who is Emmanuel; and if he
maintains that God the Word has changed himself into the flesh, which he only
assumed in order to make his Godhead visible, and to be found in form as a
man, let him be anathema.
This is not a clear union by Orthodox standards. It is an incidental union where the Logos is operating through flesh, not taking flesh onto himself as St. Cyril taught.
2. If any one asserts that, at the union of the Logos with the flesh, the divine
Essence moved from one place to another; or says that the flesh is capable of
receiving the divine nature, and that it has been partially united with the flesh; or
ascribes to the flesh, by reason of its reception of God, an extension to the infinite
and boundless, and says that God and man are one and the same in nature; let
him be anathema.
He is denying communicatio idomatum, the Orthodox belief that the properties of divinity and humanity are transfered to each other in Christ (i.e. Christ heals by using human spit).
3. If any one says that Christ, who is also Emmanuel, is One, not [merely] in
consequence of connection, but [also] in nature, and does not acknowledge the
connection of the two natures, that of the Logos and of the assumed manhood, in
one Son, as still continuing without mingling; let him be anathema.
You can't get more blatantly heretical than that.
4. If any one assigns the expressions of the Gospels and Apostolic letters,
which refer to the two natures of Christ, to one only of those natures, and even
ascribes suffering to the divine Word, both in the flesh and in the Godhead; let
him be anathema.
Again he denies communicatio idomatum.
5. If any one ventures to say that, even after the assumption of human
nature, there is only one Son of God, namely, he who is so in nature (naturaliter
filius = Logos), while he (Since the assumption of the flesh) is certainly
Emmanuel; let him be anathema.
He is denying the hypostatic union here.
6. If anyone, after the Incarnation calls another than Christ the Word, and
ventures to say that the form of a servant is equally with the Word of God,
without beginning and uncreated, and not rather that it is made by him as its
natural Lord and Creator and God, and that he has promised to raise it again in
the words: "Destroy this temple, and in three days I will build it up again"; let
him be anathema.
Again he denies the Holy Body of Christ and communicatio idiomatum.
7. If any one says that the man who was formed of the Virgin is the Onlybegotten,
who was born from the bosom of the Father, before the morning star
was (Ps. cix., 3)(1), and does not rather confess that he has obtained the
designation of Only-begotten on account of his connection with him who in
nature is the Only-begotten of the Father; and besides, if any one calls another
than the Emmanuel Christ let him be anathema.
Denies the hypostatic union.
8. If any one says that the form of a servant should, for its own sake, that is,
in reference to its own nature, be reverenced, and that it is the ruler of all things,
and not rather. that [merely] on account of its connection with the holy and in
itself universally-ruling nature of the Only-begotten, it is to be reverenced; let
him be anathema.
He is saying that Christ's body is not holy, it is only holy in a secondary sense as being connected to the Logos. This is not Orthodox teaching.
9. If anyone says that the form of a servant is of like nature with the Holy
Ghost, and not rather that it owes its union with the Word which has existed
since the conception, to his mediation, by which it works miraculous healings
among men, and possesses the power of expelling demons; let him be anathema.
He misunderstands the hypostatic union. The flesh of Christ *is* the Word of God.
10. If any one maintains that the Word, who is from the beginning, has
become the high priest and apostle of our confession, and has offered himself for
us, and does not rather say that it is the work of Emmanuel to be an apostle; and
if any one in such a manner divides the sacrifice between him who united [the
Word] and him who was united [the manhood] referring it to a common sonship,
that is, not giving to God that which is God's, and to man that which is man's; let
him be anathema.
Again he denies communicatio idiomatum.
11. If any one maintains that the flesh which is united with God the Word is
by the power of its own nature life-giving, whereas the Lord himself says, "It is
the Spirit that quickeneth; the flesh profiteth nothing" (St. John vi. 61), let him be
anathema. [He adds, "God is a Spirit" (St. John iv. 24). If, then, any one maintains
that God the Logos has in a carnal manner, in his substance, become flesh, and
persists in this with reference to the Lord Christ; who himself after his
resurrection said to his disciples, "Handle me and see; for a spirit hath not flesh
and bones, as ye behold me having" (St. Luke xxiv. 39); let him be anathema. --
N.B. This bracketed section is certainly a spurious addition and is lacking in
Even the unbracketed portion is heretical. He is positing two subjects: God the Word and the Man Jesus Christ. There are not two subjects in Orthodox Christology.
12. If any one, in confessing the sufferings of the flesh, ascribes these also to
the Word of God as to the flesh in which he appeared, and thus does not
distinguish the dignity of the natures; let him be anathema.
Again, two subjects; heretical.
Now let us examine some excerpts from his writing, The Bazaar of Heracleides
He voluntarily practised obedience as a rational nature, with thought and with examination, and with the choice of good and with the refusal of evil.
He is giving the possibility of Christ sinning as a man here.
He took a nature which had sinned, lest in taking a nature which was not subject to sins He should be supposed not to have sinned on account of the nature and not on account of His obedience. But, although He had all those things which appertain to our nature - anger and concupiscence and thoughts - and although they increased with the progress and increase of every age [in His life], He stood firm in thoughts of obedience.
In other words, Christ is God because he "finished the job." This is not only Nestorian but partly adoptionist!
He suffered all human things in his humanity and all divine things in the divinity; for birth from a woman is human but birth from the Father is without beginning, whereas the former [is] in the beginning, and the one is eternal while the other is temporal.
Again, denying communicatio idiomatum.
God is one thing and man another; but Christ is not one thing and another, but one in prosopon by union GÃƒÆ’Ã¢â‚¬Â¡Ãƒâ€šÃ‚Âª for they took the prosopa of one another, and not the natures.
In other words, the union is incidental and by will, not by nature from all eternity!
The natures subsist in their prosopa, and in their hypostases and in the prosopon of union. For in respect to the natural prosopon of the one, the other also makes use of the same on account of the union: thus there is one prosopon of the two natures.
I'll give it to Nestorius that at this time the terminology of prosopa was not clear, so we can't be too harsh with this passage. However, despite that, I will still criticize it as it suggests two persons in a union of will, not nature.
The prosopon of the divinity and the prosopon of the humanity are one prosopon. The one on this hand by kenosis, the other on that by exaltation.
Heretical as usual. He is saying that Christ's nature is divine because of the Passion.
We understand neither that which took nor that which was taken in distinction, but that which was taken in that which took, while that which took is conceived in that which was taken, for tht which took, therefore, is not conceived of itself, nor again, that which was taken.
This is the passage that some use to try and rehabilitate Nestorius in our day. But taken in context, we know he is not saying that there really is a union of nature: only that from our perspective there is no difference.
As in the Trinity there is one ousia of three prosopa, but three prosopa of one ousia, here there is one prosopon of two ousiai and two ousiai of one prosopon.
An ambiguous passage that I think can be taken either way.
Nestorius thus, it can be shown, did not teach an unconfused union of the Logos who took flesh as Orthodox do. You said that your original post did not address the mechanics but I think that Nestorius' quotes show that he just simply did not teach any kind of union as we know it.