Destiny's End is right, since Riku is still able to use the Keyblade the first time he and Sora meet up again, not necessarily fight, and at Hollow Bastion when he takes it from Sora.
As for the second part, Ansem wanted to possess Riku since he wanted more power than Sora. He was easier to manipulate in a way and I guess cause Sora had the Keyblade (possibly a stronger heart) and it probably repelled him like a bug. Or he wanted to possess who was supposed to be the Keyblade Wielder is also my guess. Type of answer you're looking for?