I enjoyed her portrayal of Rand but I felt she had little to work with. The character itself was not a good one in my opinion. But this is no fault of Grace's, the problem was in the writing. I just don't think she was a compelling character.
I am not really sure if the big 3 would have been Kirk/Spock/Rand but I do think both Grace and Deforest were thought of as larger supporting characters than Sulu, Uhura, and Chapel in the 1st season. There is obviously a lot of Kirk/Rand in season 1 which was dropped for more Kirk/other women.
As for Grace being let go what I read was she was sexually assaulted by a paramount executive. No word on who this executive was. Clearly it was extremely hard to be a woman in the business back then.
Anyways Grace was making the rounds in the unemployment lines when Deforest Kelley spotted her (even he had to pick up unemployment checks after Trek was over) and apparently told her fans were asking about her at conventions and so she started to attend and I guess came back into the Star Trek family.