None of what you say belies the fact that the secretary's grotesque letter--the one purportedly composed by the Archbishop in which he owns up to his own supposed "errors", makes no mention of any definite date. The "iffy" tone of it underscores the truth of what Lefebvre maintained. It reduces what should have been something definite into something merely proposed. It also leaves the name and date up to the Pontiff--maybe--if the Pontiff so wishes! You prove my point! There is no mention of August 15. If it had been as definite as you suggest, where is mention of it?
You talk about the Archbishop's memory. In fact, it was pretty darn good--though he was speaking informally, without letters at hand. But he got the jist of what was happening--he was being given the runaround. The secretary's final letter proved this--and made it clear all the rest was to string him along. But the matter was too important for this to be left up in the air and not pinned down. The Pontiff was hostile to Tradition and would know a loophole when he saw it--just look at how he betrayed the FSSP ten years after his motu proprio promised the Fraternity its priests might use the '62 missal exclusively.
Come on. It's a form letter which was supposed to allow both sides to think they won - Msgr. Lefebvre apologizes for his "mistakes" which he "may have" he committed in good conscience and he forgives the Pope for what he suffered (which was very little in the Pope's mind):
Most Holy Father, it is this good of the Church that I have pursued in all conscience in the sight of God during these past years through much suffering. However, I know that even in good faith, one can make mistakes. Therefore, I humbly ask you to forgive all that in my behavior or that of the Society may have hurt the Vicar or Christ or the Church, and on my part, I forgive from the depth of my heart what I had to suffer.
There is no mention of August 15. If it had been as definite as you suggest, where is mention of it?
The August 15th offer by Ratzinger was after Msgr. Lefebvre rejected the letter. The conversation related by Msgr. Lefebvre in "Fideliter" took place after May 20th, while this letter was referred to in his Conference less than a week after May 5th.
just look at how he betrayed the FSSP ten years after his motu proprio promised the Fraternity its priests might use the '62 missal exclusively
You must be referring to the decision that all clerics of the Latin Rite have a right to say the Novus Ordo. But this has nothing to do with exclusivity of the Fraternity - they are perfectly capable of saying only the 1962 Missal, as they cannot be forced to say the other one. "Ecclesia Dei" did not promise that the Fraternity would be stripped of the right to say the 1970 Missal, either.