I'm in search for the same white whale. There's quite a bit of documentation for multi-seat configurations for Linux, i.e. supporting the use of multiple screens and keyboards for separate simultaneous logins.

However I'd like to remote into a separate game scope session with its own human interface inputs and virtual audio and video outputs, as the same primary user normally logged in active desktop environment session. I'd like it so the remote and local sessions would not interfere with each other state, but without necessitating multiple Linux users for each session use case.

That last bit is what makes it more tricky and very niche. Supporting essentially multiple desktop environments probably demands separate user debus sockets. Using c groups via containers makes that viable, but like you, I also like to avoid containers and extensive volume mounts.