I wondered about Daffyd, too. I used to think it was more "dah-vid" sounding after looking into Welsh a little (and after working with a guy in Cardiff who's name was "Dafid" - his surname was Llewellyn, incidentally :P - and they pronounced his name as "Die - vid"). But the key to "Daffyd" is the double-f/single d.
In Welsh pronunciation, single f's have a v sound, double f's still have an f sound; single d is a "d" sound, double d is the "th" sound - so "Daf-id" is fine/correct...I think...
Guide:
~Link~But, Brydda's dad's name - Grufyyd - which I know I want to say is "Gruff-id" is probably meant to be pronounced "Gruh-veed".
Edit: and Gwynedd would be "Goo-en-ith" (say the 'Goo' part fast, though). Sort of like Gweneth, with more...roll, if that makes sense.
Edit 2: But then using that logic, Brydda should be "Bree-tha" (shorter sounding double-e though), but it can't be (can it?) because Dragon calls him "Birda" at one point, when she mispronounces his name (more likely they call him "Bree-da" than "Bree-tha", then). So I don't know if we need to use the Welsh pronunciation as a guide to begin with :-/
Don't even get me started on how Llewellyn should be pronounced :P too much hacking/phlegm involved.