Re: Are there any other potential cures??
Seems like you are using the method described in Apple Support's linked article, without success. The online article uses the way things are set-up to work:
How to: Move or copy mailboxes in Mail on Mac - Apple Support
You might want to check with the experts on Apple's Mail Support Team:
There is a "Get Support" link (eg: online chat / phone / email etc.) at the very bottom of their website (link below):
Mal Support website: Mail - Official Apple Support
___________________________________________________________________________
Re: "It would be great if they added an 'edit mailboxes' option with a right click/other like editing safari favs etc".
If your MacOS is up-to-date with the newest compatible version for your Mac, and Apple's method is not working for you, you might want to send feedback, (and / or a feature request) to those who work behind-the-scenes at Apple. They will likely not reply, yet your feedback will be carefully read and logged, and if others are also experiencing the same issue, we may see an update that addresses it.
Web-form for feedback: Feedback - Mail - Apple