Quo Vadis,
Two things lead me to believe the letter was genuine and not obtained by deception.
1.) Cardinal Ottaviani was also blind in September of 1969 when he signed the intervention, a mere 5 months before the letter to Gerard Lafond. Does this mean the Intervention is of dubious authenticity? In fact he had been blind since VCII. Are any of his other letters suspect due to his blindness?
2.) If Cardinal Ottaviani lent his signature to the Lafond letter mistakenly and through deception, why did he not disavow it at any time from February 1970 until his death in August of 1979?
As for the Mass of Paul VI being liturgical "innovation", Paul VI himself admits it twice in the address I cited! But he sees it as a good innovation. He also assumes, as do most Catholics, that the Pope has universal jurisdiction over the Mass a disciplinary matter and may make such innovations.
Fr. Cekada is entitled to his opinion, though being a sedevacantist, one naturally takes this into consideration when evaluating his opinion. Fr. Cekada seems to assume the See is vacant and then makes his deductions from this premise. Nevertheless, the words of Paul VI, as pope, in interpreting his own Mass, is a little bit more weighty than the opinion of Fr. Cekada or anyone else regarding said Mass.
You are correct that Michael Davies is highly critical of the Mass of Paul VI. However, in his book "The Church that Cannot Fail" he clarifies that despite his criticisms of that Mass, it cannot possibly be evil in and of itself as the Church could never feed itself poison. Here he is talking of the original Mass of Paul VI in Latin said by the book. He is not talking about the optional novelies that came later (CITH, female altar servers, lay EM's, guitars, etc.)