Chess is a 1x1 game. There's not even a concept of 3rd party in such a game 🙂. The possibility of multi-way engagements is what makes balancing multi-way game unfit for the ELO system.
The only simple way I can think of to adjust for the uneven lobbies is to average the whole lobby and adjust by that. E.g., a predator in a diamond-average lobby would have <1 coefficient for positive result and >1 coefficient for a negative result; and a plat team in diamond1-average game would have the opposite adjustment, no matter how their game goes.
The problem I see though, is that it could feel extremely punishing when the high ranked team gets unlucky or runs into other team that is evenly skilled but is still ranking up.
I agree that predators in diamond games are a problem and can make the games vary in difficulty really wildly, but I really don't see a simple good solution for that. But IMO the most important function of a ranked system is to successfully make the highest calibre games possible and in that the current system is already successful. Differentiating players in that highest rank is really of lesser importance.
Also, the predator leaderboards will still be a measure of how many games they played - meh, but I'd say that's a problem for the future for now.