As an ~10 year old NixOS user I appreciate your take on image based distros. My start into NixOS was not based on immutable OS, it was survivability of OS. I was searching for NixOS before I knew what NixOS is. I wanted some system that would survive me and my constant discovery of new software. It was impossible at the time and I had to reinstall whole distro every few months. I started with NixOS like with any other software - I started using it because ... Curiosity. 10years ago there were no documentation or guides on the internet, everything was in the Nix code, i was reading the code until my eyes bled, for months. Eventually I figured out that even if i play around with kernel modules, I simply can not brick this OS.
Some time back (a year or two) I was using Fedora Silverblue on my work pc... because I thought that will be more supported in terms of software that can run on the system. Boy oh boy I was wrong. For every system package i had to reboot, package management UI was for Flatpak, to install userlevel packages I had to use some other way... And to make things worse... It died on me some day after upgrade... I switched to NixOS. Smooth sailing ever since...
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