There is no way to abuse loss forgiven for afk if the other left for over ten minutes
It would be too easy to track if your duo is always going afk when the game looks bad for you.
Yes either team can have an afk and if you don't afk you only have 4 people who might afk. THIS IS NOT AN ARGUMENT NEITHER TEAM DESERVES TO LOSE BECAUSE OF AFKS.
No it wont mess with mmr or ranks. If you win the game you get points but you definitely do not deserve to lose points or mmr just because the enemy has an afk. You didn't lose because you played you lost because you had less numbers. You can also make it so that game doesn't count for the winners or losers and simply compensate them for the time they spent by giving everyone 100 blue essence, 50 orange essence, a key fragment, a loot box, etc.
There's already trackers seeing if you sit in spawn so just adapt them to see when you refuse to group and aren't split pushing.