For me, it has nothing to do with what may or may not technically be forbidden about Spock and T'Pring's betrothal. It is all about CHARACTER -- more specifically, character consistency. The Spock I saw in TOS would never in a million years have kicked T'Pring to the curb or just blatantly disregarded what was obviously a very serious Vulcan commitment.
Remember, when T'Pring wanted out of the relationship she told Spock: "By the laws of our people, I could only divorce you by the kalifee" -- a contest where two men are obligated to fight to the death!
So if the woman wants out of the relationship, she has to basically kill a man. So I don't believe for one moment this betrothal was supposed to be some sort of "back-up plan" that either party can easily disregard if they feel like it. It's a very serious commitment and IMO it is grossly out of character for Spock to completely disregard it. I know this is a different timeline but BEFORE the destruction of Vulcan I see nothing that would have altered Spock's character so dramatically.