Dont know where you are getting this. Nixpkgs is a breeze to manage compared to apt repo. Also it does not matter if you are on nixos or non-nixos system, the only difference is that nix does not take care of services on its own. What kind of docs do you miss? Nix has its own extensive nix docs page, and for packaging you also have nixpkgs documentation page - also official and not much related to nixos itself. Also nix has quite good man pages.
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
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
I am using qemu/KVM on NixOS to run games on with quite a good performance, so that should not be a problem