I don't know if she's smitten by him, but yes, she definitely has some annoyance over the fact that Keith and Rochelle are dating. Maybe BB isn't showing the whole picture, idk because I only wtch the episodes, but it seems like Mandana keeps talking about Keith's girlfriend negatively. She genuinely seems to like Keith and he's pretty much the only one she's willing to listen to but whenever the topic of Rochelle comes up, she gets sour. Like today, during almost the whole episode, she was pissed at Rochelle LOL.
And like you said, she didn't want to switch partners. She hates the fact that Keith does so much for his girlfriend (because then she has to do the stuff too) and yet still wants to be tied to him. Like I said, I think it's because she realizes that Keith is pretty nice and accommodating as far as partners in the house go.