I know the answer to your second question. Yes they did indeed use a different voice actor for Gabranth in Dissidia.
FF Wiki had this to say:
I don't think you're weird for not liking it. It's your opinion. I personally have no strong feelings about it, but I do think it's odd that they decided to get Basch's voice actor instead of the original one. What's up with that?In the English version of Final Fantasy XII, Gabranth is voiced by Michael E. Rogers, while he is played by Keith Ferguson in the English version of Dissidia, who had previously voiced Gabranth's brother Basch.
As for the other two questions, hopefully someone more knowledgeable about the game will post and give you the answers that you need. Sorry, but I haven't faced Chaos yet, and I don't really pay attention to the stuff about Battlegen. I mostly just fight Quickplay battles.
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