C.E. Evans wrote:
As far as onscreen material goes, there's no mention of Kirk commanding any other ship prior to the Enterprise.
Incorrect. In the character's bio reprinted in The Making Of Star Trek
it says Kirk commanded a destroyer equivalent class starship. This is reinforced by a reference in WNMHGB when Elizabeth Dehner metioning Kirk having asked for Mitchell aboard his first command.
incorrect. Or rather, it's your opinion
given that there's no onscreen material to support it.
For starters, it doesn't matter if it comes from a book if it doesn't make it onscreen (fans of various reference books written by people who worked on Trek know this very well). Secondly, Dehner's comment doesn't preclude that Kirk's first command was the Enterprise
and that Kirk wanted Mitchell to be there with him. So it could go either way--either Kirk was captain of a ship prior to the Enterprise
or he wasn't--as there is no way to prove either position from onscreen material.
The only answer to the question in the OP is that there is no official answer, only our personal conjecture and opinions.