Could the word-order have been Model brigdrawst dw i, and if so would fodel have reverted to its unmutated form as I have written it? Or does the 'predicate first' structure not apply if the noun in question has an adjective attached to it?
I think I can answer this (but please correct me if I got something wrong here).
As far as I understand, you can have the predicate first structure if you want to put emphasis on "model brigdrawst". In that case, there would be no mutation, as the mutation is caused by the particle "yn" in the verb first structure.
If the predicate were a definite noun, e.g. "y model brigdrawst", the predicate first structure would be obligatory. In that case, we would have a so-called identification sentence, where the subject and the predicate refer to the same unique individual (I am THE catwalk model, and no one else is). Identification sentences require the predicate first word order in Welsh.