Ok, I'm back! I opened the map with RPGMaker and the events on the desk were just as TamayoMeri said, no envelope at all. I did find it on the ChipSet folder though, on a file named "genjitu beya" (there is also a "genjitu beya02" which is pretty similar but the envelope is missing among other things). I've never used RPGMaker before so I don't know if there's a better way to explore this issue further, so if anybody has a better idea please tell me, though I think it'll be some other day since I'm having a headache too. (Chronic migraines here)
As for endings go, I've only seen the Glitch Ending and the Ending?, since I'm trying to 100% the game before I get any other endings.