Did you ever find a solution? I have gone through, almost identically, the same issue as you, and opened a ticket with them.
Just want to play the game with my son, didn't think it would be this much of a challenge.
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No solution in game. We've kludged a functional situation with Discord and a single earbud, but its not ideal. We just turned in-game chat off because I cannot get it to work for him at all. I haven't read anything about it in patch notes or even anyone else mention it at all, so I have to assume it's not getting fixed any time soon.