Don't write a novel with an older, female protagonist

[Demit]

3. Friendly amendment—

It's sufficient to say Mrs Pollifax "...then becomes a field agent" full stop. Because, on who else's merits would she have done so?

"On her own merits" is a belittling phrase, much like "in her own right." It implies that a woman usually achieves something because of who she's married to, or related to, so the phrase differentiates her from the usual and expected situation.

You can always test it by asking would you use the phrase to describe a man. If the story had been about a widower who comes out of retirement to apply for a job with the CIA, would you say he then became a field agent "on his own merits"?