Sharing his mother's diary even with MGs I think is too much for him, he is quite closed personality. MGs know some of his backstory, but the diary...I don't think so.
With Zoey, yeah, this could lead to a reunion and lewd scene.
I don't know, I think it could work with all of them. With Sage, he's already trying to get her to open up more about her feelings, talk about her past. Showing her the diary would be an example of how nice it can be to share, how it strengthens the intimacy between people.
Ignoring the murky connection between Lynette's family and the Royces, Jill can definitely relate to having only objects and memories of a lost loved one. And without memories, Tremolo has the next best thing. Jill already shared a lot about Lana in 10, MC can return the favor. It might even become another point in favor of Jill's belief in destiny.
Bella already expressed her view about the diary being special, something to treasure and cherish. She's also lost someone, dead or not. If Episode 11 were to finally reveal the mystery, and depending on what is inside the locked room—I think the prevailing theory is baby's room—that's an even bigger sign of intimacy. Although this would be the most melancholic version, very bittersweet. Not exactly conducive to sexy time.
We've been shown Maya's relationship with her mom, we know Josy chose her dad over her mom, this is MC's chance to share about his mom. I think they'd be very encouraging about it, and sort of be a mix of Zoey teasing (Josy) and Bella pensiveness (Maya).