You are most welcome, and even encouraged, to use
Dulcie Angharad on a live littl'un. I'd love to think there's one running around the Earth somewhere. :-)
Both
Demelza and
Angharad are intertwined in my mind for the same reason as you. Inspired casting. (By the way, I've never seen the Anghared spelling before. Is it a variant?)
On the subject of spelling, I prefer
Siena (the city) to
Sienna (the orange-brown color). Yes, the single "n" spelling might throw off those who don't look at maps much. But on balance the crayon image in my head of "sienna" is worse. It has a very pretty sound, either way.
I wouldn't call
Siena /
Sienna modern. But I don't think it's the best match with
Dulcie either.
Bryony has the same birght and breezy quality as
Dulcie. But you're right; I wouldn't combine them.
Louisa Bryony has a nice ring to it.
So many choices. So few children. Why don't people hire us to name their children? We could have a reality show on TLC called, "What Not to Name your Children."