Would you like to say something about your research interests?Look up John Koch. He has numerous papers on Academy.edu and published works. He is a renowned Welsh linguist. I go back to him all the time for research.
Branwen as in hat is correct.I take it the reader here is the Gwyn Jones who with Thomas Jones produced the Everyman's Library edition of the Mabinogion, so his pronunciations must have high credibility. However, I would have expected a name such as Branwen to be pronounced with A as in father rather than, as Jones speaks it, closer to hat.