Re: Why didn't Chekov ever make Captain?
...Except to Chekov, who should have known better. He was a starship navigator and bridge officer who had been aboard Kirk's ship when the Captain stranded Khan there! An obvious failure in Chekov's part, perhaps punishable.
FWIW, I don't think Chekov is command material at all. I'm firmly intrenched in the camp that believes the Ceti eel left his body not because it sensed its sibling's death or because Chekov was fighting it but because it was starving to death and needed to find another host nervous system. It didn't realize Kirk was standing over it with his phaser drawn, however.
"Many things seem clever to an imbecile." --Captain Thelin th'Valrass, USS Enterprise-- "The Chimes at Midnight"