3) Majority of players only play well certain lanes and certain champs
One the best solutions I could think of is to revamp matchmaking in this way that you could choose up to 3 preferred roles ( in order of preference). And it would match people to a complete team (similar how dungeon finder works in wow). It should also track elo separately for each role separately.
Finally this wouldn't change anything to those blow-outs. The reason for those blow-outs is snow-balling, not MM.
But snow-balling is an intended mechanic of MOBAs. We all like those close matches, but we need to accept that those are rare. Them being rare is a major reason for us to like them that much.
If you want to defy snowballing, you have to fiddle with comeback options, not with MM ...