Well I think
Halcyon is usuable. But then I'd totally name a daughter
Alcyone (no, I'm not kidding. Her nn would be Alcie) so maybe something to keep in mind.
I like the story behind it. I like that
Alcyone is the name of a star. Kingfishers are pretty birds. The
Halcyon days are a nice association (pleasent weather in winter? whats not to like?).
I think
Halcyon nn
Hallie would be lovely. I met a little girl called
Halcyon once.