I have watched The Motion Picture several times and I really like this gorgeous movie.
There were some changes due the time gap, that's why one of the challenges was to get together familiar characters and to introduce some background history. But some explanations weren't enough convincing and plausible (IMO).
I know about the Lost Years books, but let's pretend, they are not exist (it's not a canon and were written after TMP, anyway).
why Kirk accepted the promotion to admiral?
I guess, there was no alternative. If you are on duty, you have to accept rules. According TMP Kirk is not happy with his promotion, but he can live with it, if there is a hope to return Enterprise. So, admiral Nogura had manipulated Kirk right from the end of five year mission. But this explanation is not quite sufficient due the Kirk's strong character and his beliefs. More likely he will simply retire from the Starfleet to save his freedom. Or am I wrong?
Why Spock resigned from Starfleet and started the Kolinahr?
There should be a very strong reason to do it. Not simply "I'm tired of my dual nature, I want to be a true Vulcan".
I guess, there are parallels between V'Ger and Spock storylines. Spock rejects his human side (feelings, emotions), and V'Ger never experienced human feelings. They both need human part and they both have to understand it.
But it seems to be very odd, that V'Ger touched exactly Spock's mind. It is not very plausible, the probability of this case is very low.
Please, any thoughts!