If the Chair is empty, that means that the visible head of the Church is no longer exists.  He is gone.  Why can't one reasonably speculate that if the head is gone, the body, effectively speaking, has disappeared, as well?  We have no pope, so we have no visible, viable Church structure either?

This is not far from Protestant thinking, in my opinion.  Luther called the pope a fraud and an "anti-Christ."  Following on that He reasoned that the entire Roman Church had no legitimacy either.   The sedes refer to the last four popes, if I am not mistaken, as "anti-popes."  "(A)nti-Christ," "anti-popes," take your pick.  These two pejoratives differ from one another but little.
I came out of a division of Protestant fundamentalism which argued that the church is invisible.  That is, it doesn't have a visible, organized structure.   Our sect's spiritual forefather, J.N. Darby,  argued that the visible church had ceased to exist centuries earlier.
The sede thinks much the same way, in my opinion, though there may some qualitative differences, one being that, unlike many Protestants, the former does not overthrow and abandon, wholesale, basic tenets of the Catholic Faith.

That's the problem -- it's not just about the "sede" (the Seat of Peter)

It's always an issue about the whole CHURCH --

So they should be called Ecclesiavacantists or some other term that implies that the True Church has gone "underground", "invisible", etc.

And THAT doctrine is even harder to swallow than their teaching on the current Pope.

Notice that even Gladius didn't address most of your points.

I agree that it smacks of the early Protestant reformers, who asserted that the Church had gone "invisible".

How many Sedevacantists have no problem with the current college of Cardinals? Do they grant them the Episcopacy, or do they deny them this, just as they deny the Pope his papal office?

Sorry, but without a college of Cardinals there will NEVER be a "true pope" elected. Hence the reason most sedes are extremely apocalyptic. They expect that St. Michael is going to personally choose the next pope, or that the world will end in a few years, etc. Others, breaking off into sub-sects, elect their own "popes".

As Bishop Williamson is fond of saying of late, "Kyrie Eleison!"

Matthew

I don't mean to put words into his mouth, but I think  by "empty 250 times," he refers to interim periods, during which, for any number of reasons, a pope's election was postponed or disputed.  That is not the case here.  We have a pope duly elected, (to all appearances), by the cardinals, and accepted by the majority of Catholics.