Those are good examples, because all three of those assumptions are veiwed with suspicion. In my field, a massive amount of progress was made by rejecting the law of the excluded middle, for instance.
AC is a good example. It was an open question before Godel proved that ZDC + AC has a model, and Cohen (I think?) proved that ZDC + ~AC has a model.
But if I didn’t know that, what would I expect a proof of the axiom of choice to look like? It would be a proof within ZDC that AC were true. If I were presented with that sort of proof, then I would claim that I had a reason to believe that AC were true, namely, ZDC.
But we don’t have that sort of proof here. We have a proof that, in ZDC plus one of the propositions that are equivalent to the axiom of choice, like Zorn’s lemma or the wellordering theorem, then you can prove the axiom of choice.
Under the assumption (G -> G), you can prove <>G -> G using Libertarian’s proof. You can also prove G -> <>G. Thus, we have
(G -> G) |- G <-> <>G
If we agree that (G -> G) is not controversial, and is true of G = “the greatest imaginable being”, then what Libertarian is doing is assuming a proposition equivalent to his conclusion, and claiming that is a “proof of God’s existence”.
This is not a “proof of God’s existence”. This is a demonstration that if you define “God” in a certain stylized way, and then assume that he exists, then he will exist.
Just as the proof that Zorn’s lemma is equivalent to the axiom of choice is not usually viewed as a proof of the axiom of choice.