Ian, while I apprecitate your Trexpertise, I think the notion that DeLancie would have 'held up' the producers for a king's ransom is really silly. I worked at Paramount during the ENTIRE Sherry Lansing run and yes, we made the the cheapest possible films during that time. But negotiations for things like this are not always bloodthursty and base.
If they approaced DeLancie to be in a feature, they would have been generous to a point and within reason, and if DeLancie's agent hadn't agreed, they would have recast and rewritten and he would have lost a featured part in a 'major' motion picture. It happens a lot when actors pull out of films.
Kirstie and her agent talked herself out of a featured role in a subsequent Trek film and they just recast. Leonard agreed to die in II and then agreed to come back with the stipulation he direct III.
I just think your theories of absolute certitude regarding these things is unrealistic.