I did too. Not a common Welsh name. I asked my friends and they hadn't heard of a Hefin. One old lady said it was an "old fashoned man's name more from the north." So you live and learn.
Is the word bridegroom still used in English in Wales? That is not a term we use in the U.S. these days.
It's usually abbreviated to "groom" in speech and casual writing, but it's still use used in more formal reports of weddings.
Because it's emphatic. You don't use the "Mae" in this case because "ydy" is the verb. You could say, "Mae Hefin yn briodfab," (Hefin is a bridegroom), but to make it clear that the bridegroom is Hefin you say, "Hefin ydy'r priodfab."