NixOS is a Linux distro that does a lot of things differently (for the greater good), and gaming is heavily Windows thing, which is hard to do even on conventional Linux distros (its getting better tho). That said as for your points:

1a. One of the things that would help you, is programs.nix-ld.enable = true; (for more info check https://github.com/Mic92/nix-ld , but its integrated in NixOS already)

1b. UE5 pull request has some things that might help (eg: you could just try running it with steam-run): https://github.com/NixOS/nixpkgs/issues/124963

2. Depends how you are building them, I am not a gaming developer nor tried to build a game without packaging it with Nix. But if you manage to run the UE5 I guess the procedure would be the same as long as you have all of the dependencies available for UE5

3. I am using qemu/KVM on NixOS to run games on with quite a good performance, so that should not be a problem

Thank you, integration with activitypub is what i was after!