Well, I've had some more thoughts on Rider's relationship to Sakura, and on what would happen with Caster.
Firstly, with regards to Rider, I'd say that Sakura would be friends with her and accept her summonings because she actually knows how to make Sakura feel good (plus, she gives Sakura anyone she needs to kill). I mean, your average summoner is likely to be a complete asshole (because the willingness to sacrifice a human life just to get laid is rather incompatible with anything else), which means that they're likely to see Sakura as nothing more than a fuck-toy, and have sex with her without regard for her enjoyment. Which, for a succubus (who is going to have a very high sex drive), isn't much fun. Rider, on the other hand, would actually treat Sakura as a partner rather than a sex toy, because she's actually a reasonably nice person, and she also knows that having a friendly succubus you can call on whenever you like does wonders for your sex life. OK, so she might be rather rough with Sakura, but she also gives her mind-blowing orgasms, and Sakura would happily meet up with her for that reason (and, also, for the companionship, because Rider would see her as a friend, not just someone to fuck).
As for Caster, I think she should have a personal grudge against either Sakura, her family or demons in general, and have set the whole thing up specifically so she could capture her (or someone similar) and exact revenge. She'd have kidnapped a bunch of people to feed to Sakura (to keep her alive and to allow her to heal her wounds so she can continue torturing her), used one of them to perform the summoning and then locked Sakura away in a magical prison, where she would torture her. Shirou would then find out about the missing people (possibly with some prodding from Rider or Rin) and go hunting for her. I'd say that Sakura would be torturd for at least a couple of weeks before she's rescued. In that time, amongst other things, Caster would use a magical spell on Sakura preventing her orgasming, and another spell that keeps her in a constantly aroused state. We'd see at least some of this whilst Shirou is going around doing other things.
When Shirou gets there, he'd confront Caster, and find out that she was holding a demon prisoner (and possibly why). Caster would probably offer to stop feeding the demon and to torture her to death, and invite Shirou to join her in doing so. He would refuse, initially thinking Sakura needs to be killed quickly but, seeing the state she's in (helpless, suffering horribly, crying for her sister etc.), he can't bring himself to do it, and frees her, taking her home with him to recuperate, and deciding to try to find a way to keep her alive without killing anyone. Not sure what would happen to Caster, but I'd like to see her end up in hell at Rin and Sakura's mercy.
Of course, Sakura is extremely horny (having been denied orgasm for two weeks), and seduces Shirou, promising not to harm him in any way. Shirou accepts, because she's a horny succubus and seducing people is her job, and they have mind-blowing sex. She also complains that she's hungry (he offers her food, but she says that she doesn't eat normal food), and he most likely offers to let her feed on him (of course, she wouldn't want to kill him). He ends up becoming very attached to her and vice-versa, and manages to get her to agree not to kill people in the future, and to try to feed on people who actually deserve it. She probably also takes him back to hell at some point, to meet her sister.
Also, aren't Succubi supposed to go and seduce people, rather than relying on being summoned?
