Question: is riku and namine a thing?
Answer: Nomura confirmed in the Ultimania that Riku picked up Namine in the end scene to act on Riku Replica's last wishes, and that he's the one Namine most wanted to see, not Riku himself. So, no. Riku and Namine aren't a thing. The "RikuNami" at the end was badly conveyed RepliRikuNami.