Eskay: I'm pretty sure the events of XY occurred in the Otherverse too, though. At least I'm counting on it for a plan of mine. In any case, I'd assume all the major WAAPT timelines are "hybrids" like you said, since placing a moratorium on using G6+ stuff in them for one person's personal theory is no good.
That being said, I do like your idea of the meteor threatening the light-and-soft-ness of the Lighter And Softer universe. Only problem is that unlike the other major universes no one in the OU knows of the Otherverse's existence, so we may just have to go with "it'll devastate some other timeline, that's reason enough."
As for Crewe, she's probably the best PC for the job, since Rayquaza wanted her to be her chosen one. :P However, I'd somewhat prefer if the summoner was a non-PC. One amusing idea I had was that Robin could do it, but I'm not sure I'd have the time. And if we go with Vyvquaza Sierra is the obvious choice.
DS: That could work too. And see my previous comment about Sierra.
My girlfriend is a Volcarona, your argument is invalid.