Ooh I love Morven, shame you can't use it. My first thought was Morwenna which is both Welsh and Cornish, St Morwenna was born in Wales.
Elowen
Lowenna
Demelza
Kerensa
Morvoren (probably too similar)
Eseld (this is the Cornish version of Iseult, and is easier to say that the Welsh version, Esyllt, due to the letter Ll in tbe latter)
Welsh names:
Bronwen is Bronwen, not Bronwyn, names ending in wen are feminine, and wyn masculine e.g. Gwen and Gwyn. Wen and wyn are often mixed up by non-Welsh speakers, hence little girls named Olwyn instead of Olwen, with Olwyn being the Welsh word for wheel...
I think Arwen was made up for LOTR, same with Eowyn (see above re: wen and wyn).
Anwen
Alwen
Alys
Iona (Yonna - eye-own-uh pronunciation is Scottish)
Olwen
Bronwen
Eira
Eirlys
Eirys
Eiry
Eleri
Teleri
Meleri
Nia
Nerys
Carys
Cerys
Ceri
Catrin
Mali
Mari
Mair
Megan
Nansi
Gwendolen
Sali
Ffion
Tesni
British Baby Names have some Celtic name lists - www.britishbabynames.com/blog/name-lists.html
Welsh names can be found in Cool Celtic, Clunky Celtic and Cosy Celtic. They have separate lists of Breton and Cornish names, but not Welsh which is a shame - Breton, Cornish and Welsh make up the 3 Brythonic languages.