Safe to leave system like DotA 2
In DotA 2 if a player leaves the game for 5 minutes everyone else in the game can leave with no penalty. This game would be so much better with this system and it would save so much time.
As for the exploits of having a smurf duoing with you and leaving for you this could be solved by punishing the leaver and the player that duoed with them or if it is a party by punishing all members of the party. That should be the risk you take when partying/duoing.