Not a letter, but his interview in "Fideliter" where he explained the consecrations. He explicitly states:
While I was facing Cardinal Ratzinger with that alternative, and while he was saying that he would give us a bishop on the 15th of August,
And why, if Rome was sincere, were the names the Archbishop proposed, refused?
Because Rome doesn't what extremists like Williamson to be made a bishop. Again, Msgr. Lefebvre said in a private letter that Rome would be willing to approve at least some members of his Fraternity and associated groups such as Dom Gerard's monastery. I note that Dom Gerard and his Benedictines have been regularized under the indult and he still publicly says that the 1970 Missal is inferior, that "this rite is inadequate in expressing the real Presence manifest on the altar, the sacrifice of Christ, the divine majesty".
traditional Catholicism would be controlled by Rome herself
It seems that the terrible Modernist plot already had entered the First Vatican Council:
Wherefore we teach and declare that, by divine ordinance, the Roman Church possesses a pre-eminence of ordinary power over every other Church, and that this jurisdictional power of the Roman Pontiff is both episcopal and immediate. Both clergy and faithful, of whatever rite and dignity, both singly and collectively, are bound to submit to this power by the duty of hierarchical subordination and true obedience, and this not only in matters concerning faith and morals, but also in those which regard the discipline and government of the Church throughout the world ...So, then, if anyone says that the Roman Pontiff has merely an office of supervision and guidance, and not the full and supreme power of jurisdiction over the whole Church, and this not only in matters of faith and morals, but also in those which concern the discipline and government of the Church dispersed throughout the whole world; or that he has only the principal part, but not the absolute fullness, of this supreme power; or that this power of his is not ordinary and immediate both over all and each of the Churches and over all and each of the pastors and faithful: let him be anathema.
You say that the SSPX isn't in schism: then you give the reason for the consecrations as staying out of control of Rome. But by these very words you implicitly admit the motive for the schism: Msgr. Lefebvre wished to remove himself from the "control" of the Modernist Rome of the Anti-Christs headed by the Successor of St. Peter.
several of its theologians for what it perceived as an affront to the conciliar Church
Care to give the "affront" committed by these theologians?
As for the issue of exclusive use of the missal, I was alluding to the ninth footnote which referenced "Quattuor Abhinc Annos,"
The Ecclesia Dei Commission has explained that the "wide and generous" application called for by the Pope removed all of the conditions except for the first:
In this regard I also wish to put an end to any further useless arguments based on Quattuor abhinc annos, Cardinal Mayer's Letter to Episcopal Conferences of 3 October 1984 when he was Pro-Prefect of the Congregation for Divine Worship. Following the interpretation of the same Cardinal Mayer when he was President of this Pontifical Commission, I wish to make it clear that, in the light of the "wide and generous application of the directives...for the use of the Roman Missal according to the typical edition of 1962" which our Holy Father called for in his Apostolic Letter Ecclesia Dei, the only condition that from Quattuor abhinc annos which this Pontifical Commission still recognizes as binding is the first, i.e., that those priests and faithful requesting the celebration of the Mass according to the 1962 Roman Missal "in no way share the positions of those who call in question the legitimacy and doctrinal exactitude of the Roman Missal promulgated by Pope Paul VI in 1970."As the dicastery charged by our Holy Father with carrying out the provisions of his Apostolic Letter Ecclesia Dei, this Pontifical Commission and only this Pontifical Commission has the right to make provisions regarding the use of the 1962 Roman Missal. No group outside of and independent of the Holy See has the authority to decide on what provisions of previous documents are binding or to rule on what constitutes an illegitimate "interchanging of texts and rites". Quattuor abhinc annos also prohibited the celebration of the Mass according to the 1962 Roman Missal in parish churches except in extraordinary cases which were to be determined by the diocesan bishop, but we note that no one is interested in insisting on that condition. (Letter of Cardinal Hoyos to Michael Davies)
But this doesn't bear on exclusivity - the 1984 Indult was never understood to prevent an indult priest from offering the Mass acording to the 1970 Missal. Indeed, since the terms of that indult were very restrictive, there were no priests dedicated only to it and therefore they would have also offered the normative Mass. "There must be no interchanging of texts and rites of the two Missals" and "These celebrations must be according to the 1962 Missal and in Latin" refer only to Mass offered according to the Missal by indult: that is, the Mass was not to be offered according to the 1965 Missal, in English translation, or as a "hybrid" with the 1970 Missal.
"You say that the SSPX isn't in schism: then you give the reason for the consecrations as staying out of control of Rome. But by these very words you implicitly admit the motive for the schism: Msgr. Lefebvre wished to remove himself from the 'control' of the Modernist Rome of the Anti-Christs headed by the Successor of St. Peter."
I would put it differently. He did not trust the modernists because it was they who were actively seeking to destroy the traditional Church. Their intentions were out in the open--the Archbishop was merely reacting to their aggressions against the faith. That is simply the truth. And the Pontiff went along with the wrecking-agenda. That is also the sad truth. Why shouldn't he have preserved Catholic Tradition at all cost? It was sanctity of the highest order on his part that he should have responded as fully as he did, at great cost to himself in terms of continual persecution.
As for the "affront" to Conciliar Rome on the part of Bisig and his theologians--suppose you tell me. They were above reproach as far as I could tell, but it was Hoyos who felt they were not thinking with the conciliar Church. For this he fired them--something Rome doesn't do very often--not even when Novus Ordo priests are caught red-handed with their hands in the till or picking up male prostitutes or raping little boys. It seems Rome only gets nervous around traditionalists--and so only their heads ever roll.