Since none of it ever REALLY happened, canon is what you want it to be. Liked a particular novel or comic book? Then it happened.
No, that's not what the word canon means. You're referring to personal continuity, and a lot of people make the mistake of thinking that's what the word "canon" refers to, but that's completely wrong. The word "canon" comes from religious usage and refers to official Church-approved texts, specifically
as opposed to texts that people not affiliated with the Church might want to believe in but that the Church doesn't count. Those texts are apocrypha, not canon.
So "canon," in a fictional context, doesn't mean "what I think is true." It's not a value judgment or a declaration of "rightness." It simply refers to the core body of work from the original creators, as distinct from derivative works. Saying that individual preference can be called "canon" is like saying that personal tastes in food can be called USDA regulations. It's misusing the vocabulary. Yes, you are absolutely free to define your
personal continuity however you want; you're completely right about that. But "canon" is the wrong term to use for that concept, because it means something completely different, despite how constantly the fans get it wrong.
^ What's contradictory about Spock having a half brother or Kirk making peace with the Klingons?
The fact that Kirk seemed to hate the Klingons again in Star Trek 6 is a big contradiction
But then, doesn't that mean the problem is with ST VI rather than ST V? You can't blame a film for contradicting something that didn't exist yet.
and why did Spock never mention he had a brother in the past?
Are you kidding? First off, Spock is an intensely private person. He didn't even tell his best friends who his
father was until he came aboard the
Enterprise. So there's certainly precedent. Second, Sybok was an embarrassment to the family and was essentially disowned/excommunicated. Spock would not have volunteered the information without need.
Besides, Kirk never mentioned he had a son until TWOK; why aren't you complaining about that?