I love
Cassandra, as long as it's prn ka-SAHN-dra (as opposed to ka-SAND-ra). I have the same reaction as you with regards to nicknames, only I really dislike
Cassie. If I used
Cassandra I'd go for Cassa or
Cate as nns.
I think
Cassandra Mary is the best of the possible combos that you've listed.