If you're not a
Maggie fancier, then avoid
Magnolia. And
Magdalen. Others will be!
Clementina is a good way to ensure the EE (rather than IE) pronunciation. But it makes an already long name really unmanageable for everyday use. I know a Clementene, which looks awful but also establishes the pronunciation very clearly. She goes by Clemmie, which is the risk you take, I suppose.
I've always loved
Clemency. With
Joy and
Felicity, it's my favourite virtue name. (For the record, I don't enjoy
Clemence or
Clementine.) Whether
Minnie would last as a nn, I'm not sure, because Clem(mie) is so well known, but it would be worth a try.