The only thing I'd be wary of is having Welsh names and people not being able to spell and say them. I think you'd be fine with the ones you've listed but perhaps Ffion could be the one struggled with due to the double F in terms of spelling. Also I think the name Arwen may be originally Welsh as there was a little old lady called Arwen in my village when I was growing up and she definitely pre-dated LOTR (I'd forgotten it was even in LOTR). Arwen and Ffion are my faves of those you've listed - they are both really pretty.
As an aside, I love the name Seren (Welsh for Star), pronounced to rhyme with "there" with an N on the end.
And I don't see anything wrong with using Welsh names if you're not Welsh! It just means your DD will have a less common name
I have one of the names from your list and it is 50/50 whether people can pronounce it or spell it outside of Wales. So it depends how much that bothers you. As for you using it without being Welsh I think it's fine as none of them are particularly out there.
I don't think it matters whether you are Welsh. If you like the name, use it. I love Carys. I don't like Ffion- I think of William Hague when I hear it because his wife is Ffion - I'm sure she's very nice but I don't want to be reminded of her husband.
If you like it, go for it! I'm also not Welsh, DD has a very similar name to one of those. People occasionally look perplexed but that's to do with it being quite unusual I think, rather than me not being Welsh!
There would almost definitely be pronunciation issues absolutely, but I think I can deal with that. I think Ffion is further down the list than the other two and thats the one most likely to have that issue. Didn't know it was William Hagues wife so wouldn't have gone there holly. One of those things that, once you know it would bother you.
Excellent idea giddy! That would shut people up anyway.
Ill mention those alternatives to DH cake. I think Arwen is the favourite off that list so far so an alternative could work.
I really loved the name Aneurin (Nye for short) when expecting DC2 but DH vetoed it on the grounds that we are not Welsh < crosses arms petulantly> We went on holiday to Wales when I was just over 7 months pg and I did wonder whether I would be allowed it if DC2 was born while we were on holiday (then was horrified at myself for entertaining that thought for even a millisecond as that would have meant a premature birth).
If we had lived in Wales for 4 years I would have definitely have seen it as sufficient criteria to be allowed to call the baby a Welsh name. So there. If anyone queries it, I'd suggest you say "Well we lived in Wales for 4 years and fell in love with some of the beautiful names. I always thought that if I had a DD I'd call her . . . " etc
Welsh names can become popular in England amongst people with no connection to the country - look at Rhys and Bethan. The same is true of Irish and Scottish names - there are no end of Siobhans and Kevins and Alistairs and Ians and Moiras and Eileens whose families aren't from Ireland or Scotland and have never lived in either country.
So I say, if you like a Welsh name, use it. Someone upthread mentioned the masculine name Arwyn, which is the origin of the name Arwen. (Changing the -wyn ending to -wen can be a more elegant way of naming a girl after a male relative than saddling the poor child with a moniker like Nigella or Donaldina.)