What part of the SV thesis that this wasn't a "true" (aka legitimate) pope don't you understand?
I understand the thinking, but not if you're going to stick with Councils are automatically infallible and the idea that "the proximate rule of faith is the magisterium".
A Council was convened and completed - this actually happened. Dispute it all you like but reality dictates it happened and is therefore indisputable. If all councils are infallible, then you have zero leg to stand on just knowing there actually was a real Council and this council being universal in it's "magisterium", by your definition includes a pope.
IF the pope was not the pope when it convened, then neither were nearly all the bishops in the world who all preach(ed) the same thing in unison with the pope - - and the whole Catholic world kept the faith for all those decades before V2 without a pope or magisterium, i.e. without any rule of faith at all. So much for it lasting till the end of time.