I can't help but wonder what would happen if you battled them with a full party/PC boxes
In regards to that, it seems that's been tested already. Apparently, the final PC box is hidden until after you've caught Xerneas/Yveltal in the event that that happens, so it's absolutely impossible to avoid catching them at that point in the game.
Hi, thanks for the congrats for my shiny Fennekin He is definitely one of the best shinies I've ever gotten. He was really worth the time I put into SRing. So keep going with your hunt, I'm sure you'll have a cute Fennekin soon.
About the 70 Pokemon rumour...I think that's kind of true and possibly false. Including the supposed evos of the pokemon already revealed, there are 54 pokemon. If you've been keeping up with the leaks, there's an extra handful of pokemon that take that amount quite close to 70. However, there hasn't yet been confirmation of more legendary duos/trios, so we might just get more than 70. It's pure speculation so far as to how many there are.