Re: Why didn't Chekov ever make Captain?
Though remember whilst only Kirk was charged with disobeying orders everyone else was guilty of the same offense except McCoy.
Don't you mean except Spock? McCoy may not have been privy to the actual plans because he was incarcerated, but he went along with them and was part of the formal hearing at Starfleet Headquarters after Earth was saved. Only Spock- based on the President's words- wasn't accused of any wrongdoing.
"I've sworn an oath to serve and protect the United Federation of Planets- an oath I will fulfill with my dying breath- for in my heart, I remain a creature of duty. I am Romulan." --Captain Sran, USS Naptown