Just saw today's episode and yep, Alain does seem to be siding with Team Flare, so it looks like I'll also be wrong about there being a rematch. Perhaps they'll even take the League trophy back from Alain, or he'll feel remorseful and undeserving and give it up on his own. That wouldn't be a completely legit way for Ash to finally get a league victory, but beggars can't be choosers!
It would actually be interesting if he got an honorary win from the League by defeating Alain to stop Team Flair and thus get the chance to challenge the Elite 4 that way (I think that's how this all works?). But it better be a really epic win if they want people to get over the league ending, otherwise it might make people even angrier :P