({so, i'm kinda wondering, the Z moves kinda do the same as the mega stones right? why aren't there side effects from those? also, i think the key stone balances things out mostly, or else they couldn't revert back to the nonmega})

bevendre:

In a sense, but all that a Z move does is magnify what’s there for a single burst of power, regardless of whether the Pokemon would naturally be able to sustain such a thing.  It wouldn’t surprise me if there were side effects of some sort that will be brought up in later games, that tends to happen with Pokemon.

The Key Stone is required to achieve Mega Evolution, and from all indications is naturally occurring but inert without input from a human.

Well the keystones are from cosmic radiation left after a meteor hit the planet. So you’re basically blasting them with radiation to give them super powers.