Lavinia I quite like. I have always had a love for Lovinia and
Lovisa - so
Lavinia is totally usable to me. I don't get the frump at all, but I have never met one... so that could affect my perception.
I think it falls into the same category as
Virginia to me. It can bee seen as old and ugly or retro-chic. I personally go for retro-chic.
Benedetta is decent but not my style. It is a pretty popular surname where I live and the family has a not so nice reputation.
Leocadia looks a bit like it is truing to hard. I happen to like
Leontine, Leonila,
Leonora and
Leonie. I am sure in some accents it sounds smooth, but in my accent it sounds disjointed at the 'c', and a bit harsh.
Magdalene I love, but I pronounce it mag-dah-leen. I really dislike the pronunciation you prefer, simply because it sounds wrong to me. That is a personal preference though,a nd if that pronunciation is natural where you live or in your culture - than I say it is fine. I like the name regardless.
Helle is lovely, but only if you live somewhere in which the name would be pronounced OR you are Danish. I think a person could pull it off if they were totally foreign to the Danish culture - but it would seem a bit odd. It is such a pretty looking name though, and simple too.