Your question of "after finishing the supertask, what is the probability that 0 stays in place" doesn't yet parse as a question in ZFC, because you haven't specified what is meant by "after finishing the supertask". You need to formalize this notion before we can say anything about it.

If you're saying that there is no formalization you know of that makes sense in ZFC, then that's fine, but that's not necessarily a strike against ZFC unless you have a competitive alternative you're offering. The problem could just be that it's an ill-defined concept to begin with, or you just haven't found a good formalization. Just because your brain says "that sounds like it make sense", doesn't mean it actually makes sense.

To show that ZFC is inconsistent, you would need to display a formal contradiction deduced from the ZFC axioms. "I can't write down a formalization of this natural sounding concept" isn't a formal contradiction; the failure is at the modeling step, not inside the logical calculus.