heyo! Yeah things got really cool in the Unova region. That's where the mega evolution started. Yeah, when Ash Geninja takes damage, Ash kinda feels it too. In the entire series, he actually fights of other trainers who have megas but he uses something else. Some people call it assimilation.

Yes mega evolution is temporary. The Trainers will need a mega stone for it to happen and not all Pokemon can mega evolve. Also, Mewtwo and Charizard have 2 different megas. Depending on the mega stone. :D

It Pokemon with a hint of Digimon