[levythe]

Wakaremasen would imply that you are incapable of or forbidden from ever understanding. It would be very strange.

I've honestly never heard anyone use ga before hanaseru. It's always been wo that I've heard.

[jrockway]

I have asked my Japanese friends about this and they are usually convinced after discussion that "ga" is the correct particle to use here. But "wo" sounds fine.

I think it is very much like "was/were" in English. "If I was to give you a cookie..." / "If I were to give you a cookie..." "were" is "correct" (as though there is such a thing in English), I think, but "was" sounds fine. "Was" looks pretty abrasive in writing but say it out loud and you'll notice that you hear it a lot.