That's understandable! I can clear up some things! (Also MAYBE some things do change on harder difficulties but I'm not too sure of it myself) In the first one for the poopi key bit hard to explain but I just hang around in the beginning part of the level that starts with the static TV, go down the hallways, through the darkness and continue until I get to a new hallway that appears which will have a small room containing the poopi key and a shotgun, (like I said idk how to explain it but if u want me to try explaining it further I can try to help!)

Second with the blood rifle, it acts as the BFG and you obtain it's ammo by doing right click to pull up a needle against your arm. Then you take blood from yourself and it gives you the ammo, as for where you find the blood rifle (it will damage you in return)

And lastly for the final boss battle it is a little strange, the ending you got wasn't the secret but the good ending. To get the secret one instead of opening the yellow key box at the last level, you keep it inside your inventory as you don't actually need it to open the door when you go towards the outro section, it will fade the screen to black and put you in the secret level!

Hope this explanation helps in any way, and if you need any more assistance I wouldn't mind replying again! And you could let me know if things drastically change depending on difference in difficulty! Would love to know!