From my point of view if Bliss and Petal have been directly and sexual manipulating Rose, it kind of undermines the idea that because she is a sub the MC needs to take charge and get her back, so to speak. If Bliss or Petal have already made Rose their sub and then they just give her to the MC, basically, she isn't getting back with him because she wants to be back with him and because he wants her and has taken her back. She's with him in that case because her real dom, Bliss and/or Petal, is making her be with him because it's what Bliss and Petal want. Rose has to want the MC for there to be a real chance for the plan to work, and for that to happen he has to be the one to take her, to make her his.
Now, I believe that Bliss and Petal have likely been doing some manipulation, but they would be far more subtle than having sex with her. I think they would talk up the MC, remind Rose how great a man he was and things like that to make her pine for him so when he steps up, she'll be ready and waiting. Petal and Bliss are trying to get the MC to be a dominate man for them and Rose and other women. That's why Petal worked on him the whole week. Bliss doesn't need to have gotten Rose ready like Petal did the MC. Rose is a sub (if Bliss is correct that is) so she is already pretty much primed and ready for someone to take charge, the girls want that to be the MC not themselves. Bliss wouldn't need to seduce Rose to have sex with her in order to "get her ready for the MC," the way Petal seemed to need to prime the MC. Bliss would actually need to do almost the exact opposite of seducing Rose. She needs to make Rose think there is no one that compares to the MC, no one that can please Rose and take her like he could if he were just back in their life...
But that's just how I see it. It, to me, would be odd for the girls to have sexual used Rose. It undermines their own plans and undercuts the dominance of the MC in many ways. Could be wrong, just some of my thoughts.