I'm sure that previously I've heard that she wanted to leave to pursue a movie career. Obviously this didn't really pan out, and then with TNG getting really popular in the third season I imagine she realized that she'd made a mistake, hence her attempts to get back on the show.
Diana Muldaur - that would be interesting, to see whether she'd participate. I don't think either side were particularly happy with the character at the end of the second season. Muldaur cited a lack of creativity and invention, whilst the production cited a lack of chemistry with the rest of the cast. To be fair, the second season was blighted by the writers strike, so a lack of creativity can be forgiven here I think. In terms of the chemistry, a large part of that was down to the way the character was written. They wanted a female McCoy, cranky, and wanted the kind of relationship with Data that Bones had with Spock. Unfortunately it just came across as being mean-spirited, which meant the fans really didn't latch on to her at all. Personally, I think they were right to get Gates back.
I really do agree with your assessment of the value of these documentaries. At an hour and a half, and together with the half hour 'restoration' one as well, and the other goodies, you certainly get your money's worth here. I hope they keep these going all the way through the season sets.