Does winning with a higher elo duo increase mmr gains?

It makes the people you play against better, if you perform well it will but, in return whoever you are duoing with will take a large loss in mmr and lp if you guys lose. So it really only benefits the lower player.

thats actually not true, it harms both players since playing with a higher elo player reduces the elo/lp gain so its basically a lose-lose scenario

Well which one is it lol

Short answer: yes

It averages your MMR with the higher MMR duo and places you against people who are between your MMR and theirs.

If you lose, then your duo loses more MMR as a result. If you win, then you gain more MMR as a result and your duo gains some, but not as much as they normally would solo.

It's purely beneficial to the player with lower MMR while harming the player with higher MMR regardless of the scenario. Kind of sucks, but that's just the way it is. The only benefit the higher MMR player in your duo has is that they should be more likely to overperform in games since they are playing at a lower MMR than their average.

Yes, If you are Gold 4 duoing with your friend who is Plat 3, if you win your MMR will go up more than it would if you were solo while your friend's MMR will go up, but less so than he would if he was solo. The opposite goes for losing, if you lose then your MMR will drop, but less so than if you were solo and your friends MMR would drop more than if he were solo.