Not quite knowing if Paul VI was Pope, and an uncertainly on the Novus Ordo (he concelebrated one with a bishop sheltering him) cannot really count against the Thuc line. It was early days then. A new Pope could have reset things. I think no proof has been provided that +Thuc who had seen his great brother, the only hope South Vietnam, and the war effort itself, had, murdered, and lost his country, was mentally unwell; strained no doubt, but of sufficient mind to consecrate. Mistakes were made, what with Clemente Domínguez y Goméz and his Palmarians (all Conclavists have been exercises in pure madness), but it would seem like a desperate effort to keep the Church alive. Bp Williamson has offered balanced and pastoral words on this. Prayers for all involved.