As a Welsh speaker, this is correct.
The bit about Cwnstabl turning into Gwnstabl is technically called 'mutation', it's supposed to make a sentence flow more smoothly. There are rules about mutations but I don't actually know them, it's just natural to me!
Like many borrowed words in Welsh, although Cwnstabl looks odd to an English speaker, it is pronouced the same as "Constable".
Throughout the pandemic, one of the high spots was that the Welsh for coronavirus was.... coronafeirws! Which, of course, is just a translation of coronavirus into Welsh phonetics. That's what we do with borrowed words, and is why they sound the same although they look very different!
'Prif' always comes before the word that is being referred to as the 'Chief' or 'Highest' - for example, a university is a prifysgol (ysgol means school).