Wow! I think this is the first time I've seen you recommend the removal of plants! Usually you're the one pushing for more plants in a tank
I understand why the difference though - only teasing!
Really glad you figured out the issue and managed to catch the bristlenose, wow, the damage he managed to do! This was a really educational thread, thank you for posting and updating it. Sorry about the angels you lost though.
For any future issues, might be worth considering using a food grade large plastic tote as a temporary quarantine tank for if you need to isolate a large fish. Some can hold 50g or more, just reinforce them a bit by tying rope around them and across the top to hold it steady. Add a heater in a heater guard sitting on some stones or something and a sponge filter, some plant trimmings you don't mind losing, and you're set. Then you're not setting up a second large tank, landlord need never know, but you can isolate and treat a fish in that tote for however long you need to. Just thought it might be useful to think about for any future troubles. But glad you resolved your problem!