I think I share more of Buffalobiian view on Setsuna, manipulative feels like abit too strong of a word to describe her if nothing else because it has more of a negative connotation at least for me. Though thinking about it her actions does give mixed feelings, the fact that she honestly told Touma her feelings and tried to get Touma to admit her feelings could be seen as one of the best possible action depending on how Touma would react. But her calling Touma a friend all the time and going as far as calling Touma her best friend makes it hard for Touma to admit those feelings since she seem to treasure Setsuna as a friend as well. Anyway the point is if that's why Setsuna told Touma about her feelings because she was aiming for that outcome then yes I could see her as manipulative but otherwise no.
But otherwise the one that has been flaunting the relationship the most is Haruki. Setsuna hasn't really gotten that cozy with Haruki while Touma has been around and seems to want to keep things on the friendly level when all three are together, instead it's Haruki that initiates all the skinship. Then again it's understandable that he does.
Also I'd say the underlying motive is that she doesn't want a repeat of what happened with her other friends since that seemed to have left her quite hurt and scarred, perhaps that's why she's as perceptive as she is.