Questions about ELO
 

Say i stop playing for a few weeks, would my ELO start to slowly decrease or stay the same?

Also say someone extremely high ELO get matchmaked agains people quit significanty lower than him (example here not saying it can happen) and he wins, is threre a point where he won't gain any elo for the win or would it always raise it by 1 minimum.

Thats all for now and thank you in advance for answers if recieved.

Your elo should remain the same unless there is something we don't know about in the matchmaking code.

I think you will always gain SOME elo for a win, even if it was almost certain. It just wont be anywhere near winning against someone your own elo.