Did DiZ want Roxas to leave the Organization?

Yes. DiZ planned to stop Roxas when Sora's memories were beginning to be restored. DiZ wanted to foil the Organization's plan, so he ordered Riku to stop Roxas. DiZ then put Roxas in a virtual Twilight Town to be safe from the Organization. DiZ wanted Sora to become whole again so that Sora could defeat Organization XIII, but this was impossible if Roxas was still active in the Organization.