I think that Namine still had to help. In the kh manga, she watches over Sora, Kairi, Rike, ect. praying and helping them when she can. Kairi didn't know who Namine was a t first. If she joined with her, Kairi would not understand what was happing. When they do join togetherm Kairi knows more about Namine and who she is. She also wanted to keep her promise and see Roxas again and "talk abou't everything" likle they said in teh begining of the game. Plus if she joined with Kairi then, she woulnd't have been able to open a portal back to the islands.
Roxas and Namine have hearts cuz of Sora and Kairi. As long as a nobody's sombody is still alive and not a heartless, than thay have hearts. I read in the kh II manga, that nobody's do have emotions and feeling, like when Namine cried in com or when Roxas went beserk on the comnputer. It's just that they always have this nagging feeling of not being complete.
