From what i herd it wasn't Alley herself that asked for a massive increase in salary for ST III it was her agent. Nimoy was really happy with Alley and she loved being in STII. Paramont did not even counter offer for STIII. I don't know about STVI, but I could imagine after being not signed for STIII and finding out that she would have been a traitor I could see her passing, but I cant confirm that or anything.
The sadness was that they did not add the sequel addendum to her original contract. That is 101 for new stars.