I believe the 1962 Missal is valid. I cannot see why it would not be.
I am sympathetic with the view that the changes in the 1962 Missal were not wise, and I have no objection to those priests and congregations who use a prior Missal, but I haven't even heard any of these priests suggest that the changes in 1962 were invalidating.
I know priests who use the 1962 Missal, the pre-1962 Missal with the changes in Holy Week, and priests who use the Missal prior to the Holy Week changes. Not one of them have warned me against attending Mass at another's chapel.
Even the 2008 Missal (that's the one with the new Good Friday Prayer for the Jєωs written by Benedict XVI) is even valid. The best objection for attending these Masses is the questionable orders of the priests who offer it.
(By the way, after conducting numerous internet searches, including of the Vatican's own website, I have come to the conclusion that the conciliar church has never actually promulgated this new Good Friday prayer. The only actual (quasi-official) announcement of this prayer I can find is press release by the Vatican Secretary of State. Even by Conciliar law such a change would have to be published in the Acts of the Apostolic See and it appears that no such "Act" has ever been promulgated. If I am in error, I'd appreciate correction.)