1) No. Number One and Spock were created for the first pilot.
2) No. The Captain and Doctor charcters also survived with name changes.
3) No. His are just the ones that offer the best opportunities for an episode. And we only see them in one episode. We meet Kirk's son in TWOK. And of course we meet his parents in ST09.
4) No. I don't see how that makes him the main character. We hear alot about Kirk's past too and that is the impetus for many episodes.
5) Does he? Kirk changes too. The guy we see in Season One is different than the one we see in latter TOS and in the films.