Ok, I think I may have figured it out.
So I got the option to check through M's stuff, and eventually her journal, by somehow triggering a scene where MC wakes up in the middle of the night hearing M masturbating again. The way I THINK this triggered was, I went home in the Evening, DID NOT bathe with M, passed by time by eating, then went to sleep at Night. That was the exact sequence of events I did before the second masturbation sequence triggered, and I think the important thing is to leave M alone in the bath during the Evening, and going to sleep at Night (NOT Late Night), as I suspect she's been wanting to get off there in the bath with MC, but because MC wasnt there, she has to do so at night while he's asleep.