Games =/= anime though. I mean why should he get a Key Stone that early anyway, especially since the writers may have a change of heart about Ash using Mega Evolution later? It could all be for nothing like the GS Ball.
I agree that Satoshi does not necessarily get a mega evolution. But if he does get one then I think he better obtain the Key Stone in Shala City since this is where Corni's gym from. So Satoshi can get a Key Stone during the Corni's arc.