This was amazing!!!! You've earned yourself a fan and an eager future collaborator, should I be so lucky. The core mechanic's a very clever iteration on the theme, even if I quibble with the labels (in no video game is easy mode a hugbox pocket dimension that constantly heals you, likewise in only very few is it completely impossible to heal in hard mode). I'm impressed with how well it managed to tell a complete if rudimentary story within these time constraints. I really dig the aesthetics, everything oozes style, from the sideways slant grip on the gun to the tonal non-speech vocalizations of the final antagonist.
I did run into what I think was a bug, which was that when I first encountered the boss, he spawned like a hundred minions that merked me instantly. I hit continue and not only were there no minions on the second try, but the boss didn't seem able to attack me and died in relatively few hits. Both times I was "stuck" in Hardmode, which I think was intended. Also, during the final cutscene it zooms out to reveal that you're just arms and have no body? But for all I know that's exactly what the dev intended.
Add in a few more guns and more levels and you've got the basis for a really cool FPS here.
I felt very sad when I realized that the hideous monsters I was slaying in one branch of reality were adorable harmless animals in the other.