Is Antarctica self-sustaining?

No, but there's also little we gain from trying to make it so. We know how to operate all sorts of things in Antarctica, and anything else we bring there just... needs to be operated pretty much the same way.

Finding out we can't, yet, would be pretty important.

That's the wrong approach, though. "We can't" is the default state. The thing we need to find out is how to make it so. The goal is to be able to say, after all the finding out, that yes, we can now.

Not dying just because we can't yet is pretty important.

But scale does add difference as does all the complexity required to make that scale self-sustaining.

That's not even remotely true. For one, we have barely scratched the surface of the psychology of living in an Antarctic outpost, and already what we've seen doesn't suggest long-term viability (decades instead of months). Perhaps with more people, the problems could go away, perhaps not - there's no reason to find out on Mars.