For me it's Eleanor, Felicity or Rosalind hands down.
With 3+ syllable names a nn is kind of inevitable so it's worth thinking about what they would be.
Christabel - A bit too frilly for me. I like Christa (not Christy) as a short form but suspect it would end up as Christy anyway which is quite 80s. Would you consider Christen/Kristen?
Clementine - bit of a fad, and not keen on nn Clemmy as it reminds me of clammy
Eleanor - Lovely, classic. Nell is a more unusual and nice nn if you want to avoid Ellie.
Felicity - Lovely and has a beautiful meaning. Fliss or Flick as nn.
Gayle - I agree with a previous comment, says 80s mum to me (in fact 2 of my friends' mums in the 80s were called this).
Jacqueline - It's ok but will surely be Jackie for short which is again quite dated. How about Jocasta, Jocelyn, Jessamine?
Rosalind - Lovely, and Rose/Rosie are nice nns