1. Obviously it was not or the anathemas of Severus and Dioscoros would have been lifted and the EO s and OOs would be in communion right now.
I honestly think you're just being stubborn on this point now, because it seems clear as day to me. The capitula says that the Holy Fathers taught the miaphysis formula under a certain meaning. If the Holy Fathers taught the miaphysis formula and their meaning behind it is clarified, then why would it be unacceptable to use it in the exact same way? Further, the capitula anathematizes those who use the formula if they do not understand it as the Holy Fathers did, implying that if they do use it as the Holy Fathers did that they are not anathematized.
Severus and Dioscorus were not anathematized because of their use of the miaphysis formula, but rather because of their rejection of Chalcedon and their perceived Eutychian leanings.
2. Whether he did or not the Fathers anathematized him for it.
Show me where the Chalcedon/post-Chalcedonian tradition attributed a mixing of substances to Dioscorus.
St. Cyril is the only one they admitted used miaphysis in a way consistent with diophysis. They did not admit this of Dioscoros and Severus which is why they were anathematized again at the 6th Council.
The Definition of Faith of the 6th Council, the Third in Constantinople
The holy, great, and Ecumenical Synod which has been assembled by the grace of God, and the religious decree of the most religious and faithful and mighty Sovereign Constantine, in this God-protected and royal city of Constantinople, New Rome, in the Hall of the imperial Palace, called Trullus, has decreed as follows.
The only-begotten Son, and Word of God the Father, who was made man in all things like unto us without sin, Christ our true God, has declared expressly in the words of the Gospel, “I am the light of the world; he that followeth me shall not walk in darkness, but shall have the light of life.” And again, “My peace I leave with you, my peace I give unto you.” Our most gentle Sovereign, the champion of orthodoxy, and opponent of evil doctrine, being reverentially led by this divinely uttered doctrine of peace, and having convened this our holy and Ecumenical assembly, has united the judgment of the whole Church. Wherefore this our holy and Ecumenical Synod having driven away the impious error which had prevailed for a certain time until now, and following closely the straight path of the holy and approved Fathers, has piously given its full assent to the five holy and Ecumenical Synods (that is to say, to that of the 318 holy Fathers who assembled in Nice against the raging Arius; and the next in Constantinople of the 150 God-inspired men against Macedonius the adversary of the Spirit, and the impious Apollinaris; and also the first in Ephesus of 200 venerable men convened against Nestorius the Judaizer; and that in Chalcedon of 630 God-inspired Fathers against Eutyches and Dioscorus hated of God; and in addition to these, to the last, that is the Fifth holy Synod assembled in this place, against Theodore of Mopsuestia, Origen, Didymus, and Evagrius, and the writings of Theodoret against the Twelve Chapters of the celebrated Cyril, and the Epistle which was said to be written by Ibas to Maris the Persian), renewing in all things the ancient decrees of religion, and chasing away the impious doctrines of irreligion. And this our holy and Ecumenical Synod inspired of God has set its seal to the Creed which was put forth by the 318 Fathers, and again religiously confirmed by the 150, which also the other holy synods cordially received and ratified for the taking away of every soul-destroying heresy.
The Nicene Creed of the 318 holy Fathers.
We believe, etc.
The Creed of the 150 holy Fathers assembled at Constantinople. We believe, etc.
The holy and Ecumenical Synod further says, this pious and orthodox Creed of the Divine grace would be sufficient for the full knowledge and confirmation of the orthodox faith. But as the author of evil, who, in the beginning, availed himself of the aid of the serpent, and by it brought the poison of death upon the human race, has not desisted, but in like manner now, having found suitable instruments for working out his will (we mean Theodorus, who was Bishop of Pharan, Sergius, Pyrrhus, Paul and Peter, who were Archbishops of this royal city, and moreover, Honorius who was Pope of the elder Rome, Cyrus Bishop of Alexandria, Macarius who was lately bishop of Antioch, and Stephen his disciple), has actively employed them in raising up for the whole Church the stumbling-blocks of one will and one operation in the two natures of Christ our true God, one of the Holy Trinity; thus disseminating, in novel terms, amongst the orthodox people, an heresy similar to the mad and wicked doctrine of the impious Apollinaris, Severus, and Themistius, and endeavouring craftily to destroy the perfection of the incarnation of the same our Lord Jesus Christ, our God, by blasphemously representing his flesh endowed with a rational soul as devoid of will or operation. Christ, therefore, our God, has raised up our faithful Sovereign, a new David, having found him a man after his own heart, who as it is written, “has not suffered his eyes to sleep nor his eyelids to slumber,” until he has found a perfect declaration of orthodoxy by this our God-collected and holy Synod; for, according to the sentence spoken of God, “Where two or three are gathered together in my name, there am I in the midst of them,” the present holy and Ecumenical Synod faithfully receiving and saluting with uplifted hands as well the suggestion which by the most holy and blessed Agatho, Pope of ancient Rome, was sent to our most pious and faithful Emperor Constantine, which rejected by name those who taught or preached one will and one operation in the dispensation of the incarnation of our Lord Jesus Christ who is our very God, has likewise adopted that other synodal suggestion which was sent by the Council holden under the same most holy Pope, composed of 125 Bishops, beloved of God, to his God-instructed tranquillity, as consonant to the holy Council of Chalcedon and to the Tome of the most holy and blessed Leo, Pope of the same old Rome, which was directed to St. Flavian, which also this Council called the Pillar of the right faith; and also agrees with the Synodal Epistles which were written by Blessed Cyril against the impious Nestorius and addressed to the Oriental Bishops. Following the five holy Ecumenical Councils and the holy and approved Fathers, with one voice defining that our Lord Jesus Christ must be confessed to be very God and very man, one of the holy and consubstantial and life-giving Trinity, perfect in Deity and perfect in humanity, very God and very man, of a reasonable soul and human body subsisting; consubstantial with the Father as touching his Godhead and consubstantial with us as touching his manhood; in all things like unto us, sin only excepted; begotten of his Father before all ages according to his Godhead, but in these last days for us men and for our salvation made man of the Holy Ghost and of the Virgin Mary, strictly and properly the Mother of God according to the flesh; one and the same Christ our Lord the only-begotten Son of two natures unconfusedly, unchangeably, inseparably indivisibly to be recognized, the peculiarities of neither nature being lost by the union but rather the proprieties of each nature being preserved, concurring in one Person and in one subsistence, not parted or divided into two persons but one and the same only-begotten Son of God, the Word, our Lord Jesus Christ, according as the Prophets of old have taught us and as our Lord Jesus Christ himself hath instructed us, and the Creed of the holy Fathers hath delivered to us; defining all this we likewise declare that in him are two natural wills and two natural operations indivisibly, inconvertibly, inseparably, inconfusedly, according to the teaching of the holy Fathers. And these two natural wills are not contrary the one to the other (God forbid!) as the impious heretics assert, but his human will follows and that not as resisting and reluctant, but rather as subject to his divine and omnipotent will. For it was right that the flesh should be moved but subject to the divine will, according to the most wise Athanasius. For as his flesh is called and is the flesh of God the Word, so also the natural will of his flesh is called and is the proper will of God the Word, as he himself says: “I came down from heaven, not that I might do mine own will but the will of the Father which sent me!” where he calls his own will the will of his flesh, inasmuch as his flesh was also his own. For as his most holy and immaculate animated flesh was not destroyed because it was deified but continued in its own state and nature (ὄρῳ τε καὶ λόγῳ), so also his human will, although deified, was not suppressed, but was rather preserved according to the saying of Gregory Theologus: “His will [i.e., the Saviour’s] is not contrary to God but altogether deified.”
We glorify two natural operations indivisibly, immutably, inconfusedly, inseparably in the same our Lord Jesus Christ our true God, that is to say a divine operation and a human operation, according to the divine preacher Leo, who most distinctly asserts as follows: “For each form (μορφὴ) does in communion with the other what pertains properly to it, the Word, namely, doing that which pertains to the Word, and the flesh that which pertains to the flesh.”
For we will not admit one natural operation in God and in the creature, as we will not exalt into the divine essence what is created, nor will we bring down the glory of the divine nature to the place suited to the creature.
We recognize the miracles and the sufferings as of one and the same [Person], but of one or of the other nature of which he is and in which he exists, as Cyril admirably says. Preserving therefore the inconfusedness and indivisibility, we make briefly this whole confession, believing our Lord Jesus Christ to be one of the Trinity and after the incarnation our true God, we say that his two natures shone forth in his one subsistence in which he both performed the miracles and endured the sufferings through the whole of his economic conversation (δἰ ὅλης αὐτοῦ τῆς οἰκονομκῆς ἀναστροφῆς), and that not in appearance only but in very deed, and this by reason of the difference of nature which must be recognized in the same Person, for although joined together yet each nature wills and does the things proper to it and that indivisibly and inconfusedly. Wherefore we confess two wills and two operations, concurring most fitly in him for the salvation of the human race.
These things, therefore, with all diligence and care having been formulated by us, we define that it be permitted to no one to bring forward, or to write, or to compose, or to think, or to teach a different faith. Whosoever shall presume to compose a different faith, or to propose, or teach, or hand to those wishing to be converted to the knowledge of the truth, from the Gentiles or Jews, or from any heresy, any different Creed; or to introduce a new voice or invention of speech to subvert these things which now have been determined by us, all these, if they be Bishops or clerics let them be deposed, the Bishops from the Episcopate, the clerics from the clergy; but if they be monks or laymen: let them be anathematized.