it sounds like you can use the QR scanner to get out of dex pokemon but i'm not sure of all the specifics
also it sounds like you can use the ultra beasts in the battle tree
Yup, I can confirm. Streamer on Youtube was using Xurkitree in the Battle Tree Singles.
So this probably means they're allowed on battle spot as well. Which sucks, since some of them will centralize the metagame. I could swallow having every second team having Mega Khan/Gengar/Mence and it was nice to have a place to use them myself without playing ubers, since they were mons you could breed and feel attached to.
Now, one could hardly feel attached to something as horrendous as most of the UBs. Also, the one mega limit helped balance megas since you couldn't simply stack many of them in a team. Now imagine facing a team full of UBs. I don't want to have to put up with that.