: Alright, if you want to work that way: : If there is no point when the density or mass of a black hole becomes so massive that it can't withstand it, does it just grow in density?

It grows in mass, but Swartzschield holes are actually vacuum field solutions. The mass of it isn't something "in" space anymore; it is instead a parameter of the space-time geometry.

Is it an endless supply of space? Where does all the space go? What happens when all the air in-between the atoms compress?

It makes a phase transition from ordinary matter to a black hole geometry state.

: Black holes haven't eaten up the universe yet, and assumable, they aren't relatively new, so, what happens when there are only atoms left? What happens when there is no space left? It doesn't expand that much.

The black hole is not comprised of atoms.