What is the likelihood that we’ll ever get a Grey version? Also, what if it took place in the past, perhaps during the Reshiram/Zekrom split or the Kalos/Unova war?

Well, the likelihood, I would estimate, is “close to zero.”  They’re done with that generation; they’ve moved on; they’re doing other things now.  Unless the remake train keeps steaming on to the point that we actually get remakes of Black and White, I doubt it’ll ever happen.  Setting it in the mythical past would be cool, but the trouble is that it does sort of lock certain things in – like, we know that Reshiram and Zekrom destroyed Unovan civilisation the last time they fought.  It happened; the player can’t win and the story can’t have a happy ending.  Unless of course they’re willing to pull some kind of alternate universe situation, of the sort implied by Delta Episode, where this hypothetical Grey version is actually a prequel to a completely different version of the Black and White universe that we’ve never seen.  That would be kinda weird.

Was Kalos at war with Unova?  I always got the impression it was a civil war; I don’t think Unova is ever mentioned in connection with that conflict.