It is impossible to exploit loss forgiven for afks if you do these 3 simple things.

(1 and 2) First if someone duos and their team is losing and their duo afks for ten minutes (This number is important as if the game lasted another 10 minutes it was still a winnable game, though with how fast games go at the moment perhaps 5 minutes is more fair now.) and this happens 3-4 losses in a row. That person who didn't afk gets a 14 day ban followed by permanent ban. This would include if you had someone on a different duo. If you really did get that unlucky it would be easy to prove with power outage data (which is public record) and receipts from cable workers. If someone just dcs and cannot reconnect loss forgiven for the 4 others.

(3) Make the duo lose all the points the rest of the team would have. This will make it harder for those 2 to duo. As the cheater rises the other will lower and eventually they cant duo. Yes it would suck if you did dc but if its not a common thing then 4 games worth of points shouldn't be that hard for you to make up after all it does not affect your mmr.

It is ridiculous how easy it would be to implement this and how much frustration it would solve. I shouldn't have to waste a game in a loss because someone decided to afk over first blood or because the enemy has 7 more kills or who cares why.