How to change/update Brand?

So I sell 3 sizes of the same slippers. All made by same manufacturer (ABC for example) who have licensed the Brand (XYZ) to make the items.

On Amazon they are listed separately. I want them listed as a variation listing.

I have created the upload file to do this but it wont work. Amazon and SS tell me its because
Size M has been listed with Brand ‘ABC’
Size L has been listed with Brand ‘XYZ’
Size XL has been listed with Brand ‘ABC Ltd’

To combine individual listings into a variation listing, Brand has to be exactly the same.

If I try to edit Brand myself I get told I cannot (how some people manage it I have no idea). If I upload a file to do it it comes back with errors either about being unable to change Brand or that I am unauthorised to list items with the XYZ Brand (I am not I have full authorisation from the manufacturer to sell these items on any marketplace and their licensing agreement allows this too).

I have pointed out the EAN’s on the GS1 website all show the items are made by same manufacturer but Amazon response is the usual cut and paste answer that they all have to be same Brand to combine into a variation listing.

So any other ideas how to change Brand?

In my eyes the Brand should be XYZ but Id be happy to just get them all the same somehow so I can combine into a variation listing.