No. I don't think Riku turned into a heartless. I thought he did for a long time, (like you did) but now that I think on it, he never lost his heart, he was only Ansem's (aka. Xehanort's Heartless) puppet. He was possesed not destroyed. Besides, Riku is way too strong to completely give in to the darkness. Remember his words? "You're not gonna use me for this!" So, all in all, he was there, fully...no heartless, no nobody, just a lot of dark powers and a confused mind.