Once an answer gets hidden, is there any amount of editing that can make it relevant?

Yes you can change the position of an answer.

When the relevance sorting first came out I played around with it and posted a crappy two word answer which got hidden and then edited that to a good one and it jumped up from hidden to unhidden.

There is one trick which someone unwittingly uses (not me) and despite informing Y!A staff it's still outstanding which means that the person concerned almost always ranks at the top of the other answers regardless of size/relevance. Hopefully this loophole will be plugged soon.

... but in short, yes you can edit an answer to make it rank higher in terms of relevance.

Yes, sometimes just adding more relevant information will get the answer unhidden. I have also seen answers that weren't hidden get hidden when a more relevant answer gets posted. So an answer can get unhidden or it can get hidden, it depends on the other answers, kind of a competition of sorts.

Yes, just add more to it that is relevant to the question asked and it will move out from under the "hidden" button. I'm not sure if I've added enough to this answer though, but we'll see. :)