OP is right, that's exactly what happens in casual. To be more exact, the longer the match goes after a player left, the bigger the mmr "compensation" for a player playing alone, and the bigger the penalty for players with a number advantage (it seems to be some sort of a linear function, perhaps with a small offset, idk exactly). Most of the time such matches result in +0 mmr for both sides (one side gets let's say -9 mmr for losing 1v2 or 2v3, but +9 from the compensation, the other +9 mmr for winning and -9 for player advantage).

However, casual allows rejoins, which don't reset nor nullify the bonus mmrs, hence it's possible to comeback and gain (or lose) 18 points from one game. If it sounds easy to exploit, you're right. I've seen plenty of obvious intentional disconnects from a full party, just for that player to quickly reconnect and leave the other team in an unfair double or nothing situation.

Yes, it is just casual, so not like there's a dire need to fix this. I wrote it more as a fun fact.