According to the pickup table, it's a Very Rare pickup item. You'd need a pokemon level 81-100 with the ability pickup and a ton of patience. Good luck
Those are pretty cool actually. I'll definitely be using some of these.
And this isn't my own to share but Saber's Sprites and Outfits topic has some pretty solid sprite edits. Nothing for Ari and Decibel yet though and surf and dive animations are missing two since it was made in episode 13.
If you don't take any step, level up or take it to the saloon, you need 33 EV-reducing berries in order to get enough happiness for it to evolve, acc to my calculations.
I say when guessing, go for legends who could help fix the region. That or the weakest legendary pokemon you can thing of. Then think of something you like even less. There's your answer.
You still need the bracelet or whatever (I haven't actually played with megas). When these will appear in Reborn, I don't know. They are not in-game yet though, so that mega stone is currently useless.