I think he might reconsider the all demons/devils are totally evil but frankly it wouldn't easily change him to jump into bed with her. although i can see the last part.
Well, perhaps not, but he might see that it is causing her problems. And, I don't think having a horny Succubus around is a particularly good plan. One possibility is that they could go see Rider, and then Rider could "relieve" her.
I would say she should definitely fuck Shirou, but possibly not straight away. Because, like you say, it's going to take quite a bit to convince him that not all devils are completely evil....