Hair color and body type are the only similarities the ex and Mary share, face and lips are different
I don't think you're gonna have to "fuck" anything up, all you need to do is convince Mary and you might have both her AND Simone, but that's something I'm not gonna do, even though Simone is actually closer to my type, but Simone strikes me as the type that will have her fun while it lasts, won't become serious about you, and once the fun is over, she's gone, that's the vibe I'm getting, but I could be wrong, but she does give a lot of flirty and naughty innuendos and she's not coy at all, so safe thing to do is keep her at a distance and just focus on Mary xD
Think about it, if Mary agrees to threesome with MC and Simone, what if she'll get the idea like "Hmm, well that's experienced, I wonder how having two dicks up in me would feel like", yeah, and you'd have to agree to that or you'd be selfish and a hypocrite and she wouldn't like that