1: Okay so I disabled the invincible mod and yeah, it was preventing the short cutscene of the character shooting up heroin. Did some testing and the code that blocks the debuffs was the debuff immunity code itself.
2: As for the weird looping thing, I think I realized what I did wrong: I added "//" to the code that turned HP damage taken to 0, as I still wanted to put healing items to use. HOWEVER, because I also didn't add "//" to the death status effect, my hypothesis is that whenever a character performs an action at 0 HP with the death immune line enabled, they're gonna keep looping that action. A certified script kiddie moment.
...Also I JUST realized that me doing all of that was rendered somewhat pointless, because the entire time I was playing, the items I was using were never consumed unless it was ammo being fired or currency being spent. Granted, I didn't know the "no skill usage" code also affected item use, but I played through an entire game without batting an eye at the flaw of my logic. Then again, resources are rather limited and I was doing everything I could on Day 1 morning...