From what I've always understood, Kirstie Alley was the first choice to play Saavik in STVI:TUC. There are two different stories on this, one is she was too involved with "Cheers" to meet the STVI production schedule and she had to decline, the other is she asked for more than what Paramount was willing to pay for her to do the movie, and the offer was withdrawn.
You've conflated two stories here.
Alley asked for more on Star Trek III
than several more established cast members were asking for. Paramount passed and, to Alley's chagrin, wouldn't even negotiate a lower price with her agent.
For Star Trek VI
was the issue as the filming dates would have conflicted.
However, her unwillingness to appear in "Cause and Effect" a year later (at most half a day's work) leads me to wonder if she simply wasn't interested in a Star Trek
appearance at that point in her life.