Riku took some part in the experiments in Vietnam, if i remember correctly. While that girl he was with turned almost into a Chiroptera, Riku was only able to hear Diva's song. He was not put under its control or driven mad. But I always thought something about him being in those experiments would play out sooner or later.

Saya is weaker because her will is weaker. Diva and all the Chevalier have serious determination (and are consciously aware of their battle experience, giving them an additional edge in tactics and such), while it appears Saya is only acting off her ingrained instincts/suppressed memories. It is interesting also to note that Saya actively fights AGAINST those abilities, not wanting to turn into 'some kind of monster', while the Chevalier and Diva seem to harbor no qualms with it.

I also like the thought about Diva, because she was made after, was a superior creation to the first one, Saya. Never considered it that way at all...