So what you are saying that you can carry as any role if you are better than opponent unless it's high MMR? I don't know if you've read threads about MMR and how people whine that they can't win with support because their carry sucks or whatever. See http://www.mmo-champion.com/threads/...e-to-raise-MMR
Furthermore, you do know there are several people in high MMR who play support that still win? Moonmeander is one. I played with him and he played Earth Spirit support and he created so much space for us. We also have professional games where we can clearly see impact of supports and how they can carry the team.
Easy mid heroes you can play are heroes like Death Prophet, Viper or DK. Simple spell usage on those heroes combined with strong laning. I think I posted in this thread about how to play Invoker mid like 2-3 pages ago.