I think I've seen Dashel, actually, but I can't remember where. I prefer
Dashiell. The other two could definitely
work, mind you; they certainly aren't ridiculous like Kymbirlee or anything like that. I just think
Dashiell looks nicer, and because I'd probably pronounce the names differently so I'd be confused about what prn. you're going for.
No alt. spelling immediately come to mind except one:
Robyn. I prefer
Robin on a boy and
Robyn for a girl, which is very weird for me since I'll be the first to say that I don't think a "y" makes a name any more feminine than if it were spelt with an "i." Very odd.