Yes, Garrett's contract expired in January. Originally, the word was that he would stay until April in hopes of taking over the role of Oscar if the show extended and Lane did not. But the original plan to have Garrett understudy Lane was not carried through to the present either. He decided not to stay...that's all...not big intrigue.
As for his eligibility, if he had stayed through January when the Tony committee is supposed to see the show, he would be eligible. If he wanted to go home to L.A. and the contract was up, my guess is that the Tony nomination just didn't mean all that much to him.
I did see him several times in the role. Thought he was quite weak the first time I saw him but he did grow into the role after awhile, though I never understood the audience reaction to him when the curtain came up...it was like seeing ROBERT from Everybody Loves Raymond in the theater and that's what the audience reacted to. I didn't think he was all that wonderful...though he was adequate.
I don't know that he would have gotten nominated anyway...there was nothing that was particularly a 'knock out' about his performance.
"Life is a lesson in humility"