Doesn't Dehner say at one point that Kirk and Mitchell had been friends for "years"? And Mitchell mentions Kirk at the Academy. I think they knew each for years.
My own bit of fanon is from one of the older novels, I'd have to flip through a few to find it, but it talks about how Kirk had requested Mitchell as his first officer, but Starfleet had assigned him Spock instead, because Mitchell was "too much like Kirk" and a captain needs an XO that can present different ideas and viewpoints (such as Spock). After reading that novel, I've never seen Gary as XO in WNMHGB, at most he would be second officer.
Hmm. Going back a few years with this one but I think it was simply titled "Enterprise". I read it way back. Kirk's first assignment, when he got the ship (and IIRC, he was in his 20s in the novel) was basically ferrying around a 23rd century circus on a tour. And Gary was actually injured and not even on the Enterprise for the entire novel.