Hey
@Tim Colling
I have some experience merging FB pages, though it's been a while. As far as I know, there is no direct support at FB - just the report duplicate as a user from the listing for unofficial pages, and the merge request link if you have Admin privileges for the pages. (
https://business.facebook.com/pages/merge/ or
https://facebook.com/pages/merge/)
I would advise claiming the unofficial pages so that you can use the page merge link - it's more reliable than the user edit option, and you can update the NAP to make it match the main listing better if need be.
...Unless (I think) your client uses FB advertising... then advertiser support may help you.