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The arguments for sedevacantism are mostly taken from Bellarmine and his theology on the papacy. That is a manifestly contra-protestant theological tradition.

Yes, true, but I think what he was getting at was not oppositional to St. Bellarmine or his theology, but the notion that laymen on their own accord would  use it subjectively/privately to declare the Holy See vacant.

Also often forgotten is that St. Bellarmine didn’t actually believe the situation of an heretical pope could ever arise, and his writing on the subject is therefore largely hypothetical (ie., Much like St. Thomas More’s “Utopia,” Bellarmine is discussing things he didn’t believe could happen, and giving his abstract thoughts for the sake of argument).

But would St. Bellarmine maintain his opinions in the real world when confronted with the conciliar papacies?

I wonder.

It also gets overlooked that St. Bellarmine’s opinion is one among several, and that the Church has not made it Her own.  This aggravates the rashness and boldness of those who would unilaterally declare the see empty.

And let’s not forget there is still an unsettled debate about what St. Bellarmine’s position really is (ie., Some say he still agreed with Cajetan/Jst/Suarez that the Church had to be involved in the process, at least so far as declaring the fact of the heresy, but opposed the aforementioned writers only insofar as they believed a second judgment of the Church was necessary that Christ had deposed their pope).

But the common sede rendition has it that at any time a layman thinks a pope has committed heresy, the see becomes empty.

It is difficult to believe Bellarmine would invent a doctrine so ruinous to Church unity (particularly over the course of three generations and seven papacies, and counting), and it is in this sense that the allegation of Protestantism is made (not against Bellarmine, but against those who wield and misapply him).

https://www.youtube.com/watch?v=Odz1fgTk0JA

I did a long interview with the Youtuber Classical Theist on my experience as a Sedevacantist and my current views.  

I only listened to a few minutes, at 3:35 you commented: "...until then, you had kind of this theory of papal positivism, where God would directly prevent anything remotely coming from there, basically the Holy Ghost would protect at all times even material [heresy] from happening....." - Fr. Fenton taught pretty much this same theory as though it is a dogma or teaching of the Church.

In your initial post, you said: "I am a former Sedevacantist who accepts Vatican II."

I wonder how much of a roll, if any, this theory in particular played in your decision to leave tradition to accept V2.