Thank you so much for the compliments and in-depth critique, it's very much appreciated! I'm really curious about this bug you mentioned, as I've never seen it before and it hasn't been mentioned by other players. Do you recall any specific instances of items that were missing besides the door switch? I know the difficulty settings will remove certain enemies and change some of the collectibles, but it shouldn't affect anything else as far as I know...
Addressing some of the other things you mentioned, I will probably power up the rifle a spec since others have said that it was a bit underwhelming in terms of effectiveness. I also do like the idea of patrolling enemies. The only problem is that they're all visually designed to be facing the player at all times like in Bad Toys, unlike Wolfenstein and Doom where the enemies have sprites for 8 different directions, so it looks more natural if they're moving away from the player. There will be a large variety of enemies though, with various attacks that should keep players on their toes! I'll have to look into the framerate situation, as far as I know, GameMaker automatically caps it at 60fps so I'm not sure what might be causing this issue.
All in all, this was a very informative critique, and you pointed out some issues that I missed like the gunshot sound not adjusting to the sound settings that I shall address. Thanks again for writing this up! And again, if you have any more information about this bug, definitely let me know!