Finally got the CG for the mom/mother/dildo degenerate in room 05 so figured I'd share what things I did in the vicinity of getting the scene in case it's helpful (
with a game this janky I wouldn't dare frame it as things I did to cause the event to happen):
After unlocking all the other scene options I gave her some love on "day 0" (basically just a reference point since I couldn't remember exactly when she'd got some love before that and didn't want to guess at whether any temporary failure of the scene to trigger was caused by misremembering) and then stopped seeing her. After 3-4 days at around 11pm she'd wake up and start wanking on her own (there's one of the pink event squares above her head when this happens). Opening the door and clicking the event triggers scene 4, which then gives you the CG after you "finish" that scene (I'm 90% sure scene 4 is always the last scene given the dialogue in the CG explicitly refers to
A few other notes that might be relevant (or might just be ephemeral, irrelevant game-jank):
- I noticed that if I hid in the bathroom all day it seemed to mess with the son coming home? As in, he didn't seem to come home on any of the days I was hiding there, but did seem to come home after triggering a sex scene with the mom (even if that scene was started by clicking the sleeping mom at like 1am or something). So my thinking is that it might be that if you wait in the bathroom every day after you stop boning her then scene 4 might not trigger (since the son seems to need to be home for it), although I didn't test that (or test whether waiting in the bathroom or even entering the apartment might "reset" the number of days you need to wait for the scene to trigger)
- On the first 2 days I broke in when she wasn't home (fuck, the kinda shit you write about "doing" when talking about degenerate porn games) and popped an aphrodisiac in her drink. The scene didn't trigger on either day, and I didn't do it for the subsequent day(s), so I'm not sure if giving her drugs a) shortened the number of days you need to wait, b) reset the timer on how long you need to wait), or c) did absolutely fuckin nothing.
- Related to the possible bug of the son not showing up while you're hiding in the bathroom it might be that you can't be in the apartment in any sense if scene 4 is to trigger. Obviously that makes it a bit harder to know if it actually has: I put a spy camera in there earlier in the game so could just watch that way, or you could just eye the door around 11pm (since her night wank triggered pretty consistently near to after 11pm for me).
- I didn't make it up to the apartment in time to click on her event box the first time (I was hiding out in my apartment in case it was another scene where you need to be there for it to trigger), though I did manage to get into the apartment just as she finished up. The event repeated the next night (I didn't sleep with her or drug her or anything between the first and second times) so it seems like once it starts the opportunity keeps popping up every night (I assume that is presuming you don't sleep with her in between, but I didn't test if that is actually the case).
Also in case it's useful to anyone else in future; I ended up figuring I should try ignoring her for a few days after rereading the text for the hint the pervert salesman gives you in the dialogue.json file (under "mom_03_complete" he mentions that she'll definitely start jilling it if you ignore her for a few days). So that might be a useful last resort for anyone stuck on something.