1) No, Mew was supposed to be available on Beta 11 but ended up not being accesible to the player, Aethestode mentioned the reason at some point of the thread.
3) I don't know if he did gave a Porygon in past Betas, but no, i'm pretty sure he never gave one in Beta 10 or 11.
It can only be bought at Celadon's Casino, afaik.
It's not like there's anything that needs fixing anyways tho.
Remember that B12 is being made from the ground up, so reporting bugs from B11 is literally pointless.
Only those bugs that actually are in the B12's Demo like the Pickup one or the one at Route 4's PCenter with Misty could have been in the Beta 12 if people didn't reported them.
Also, Aethestode definitely looks like he's wanting to add the whole Alolanmons Set to the game anyways, one or two requests here and there if they do make sense and he actually decides so, could be acceptable.
In the end, he knows what is the best for the HackROM, he's being doing everything so freaking incredibly well that it doesn't really matter, he knows what to add, what to consider and what simply discard. Let's have some faith on him.