Which came first, Alley saying no to coming back, or Roddenberry kvetching about Saavik being the traitor? One suspects that both events together led to Meyer re-evaluating Saavik's role in the script. I've never heard that he ever even considered
asking Curtis back.
The third story is that Kim Cattrall was offered the role after Alley turned it down, but she wasn't keen on simply resurrecting Saavik so Meyer retailored the script to give her a bit more freedom in the role.
Maybe the real explanation is a combination of all three events?