1. I personally prefer
Cecilia over
Cecily because I think it's a very pretty name and it is more mature than
Cecily which sounds more like a nickname.
2. I don't think
Cecilia sounds 'too catholic'. It sounds like a normal name and if you hadn't mentioned that question, I probably wouldn't have known that it was a common name given in strict Catholic families.
3.
Cecily would definitely work as a nickname for someone named
Cecilia.
4. I can't think of a nickname for
Cecily because as I mentioned in the first question,
Cecily already sounds like a nickname.