1. for most of your questions, exp system is based on console one, gen V to be exact. So your questions mostly should go to nintendo. Our game is pretty flexible tho, but i am sure there wont be any Exp base balancing since nintendo already make a good calculation. and i know many people against the change like this especially pvp players.
2. nobody told you to "not allowed" yours to evolve. It just the matter of training method, most of unevolved one also weak on battle, but level up faster due of that trait.
this is anyway, usually occur on late game, yeah, your starter and party for story dont need to do this method.
3. Conclusion, if you are here for story, dont mind this stuff, but if you are hardcore as PVP players/daycare/trader, you might need this method.
4. there is a prev thread, you dont need to make twice, just go there and discuss. thread lock.