1. I used to prefer
Susanna, but lately I've been leaning more toward
Susannah. The H gives it a more antique look.
2. Eh, it would probably come up at some point in her life, but I don't think it's a bad association or one she will particularly resent. I wouldn't let it stop you from using it.
3. I like Susanna(h) much more than
Olivia.