I meant to report this one a long time ago, but kept forgetting.

In my first playthrough, Ronnie Shaw never became available as a trader after the Old Guns mission. She counted towards the population of The Castle, but could not be used as a trader or assigned to anything. She would simply repeat her stock lines over and over if interacted with.

I'm 90% certain I know what the main causal factor of the bug is, now that I've played that mission a couple of times since then, successfully.

At the conclusion of The Castle's leg of the mission the player is given a prompt to speak to Preston Garvey one stage too early. At that point, there is still one mission-specific line of dialogue required from Ronnie Shaw to activate her as a proper settler. If the player leaves and speaks to Preston, concluding the mission (as I did the first time) that line of dialogue is no longer available, and Ronnie Shaw cannot activate properly as either a trader or settler.