1. I like the name
Ginevra and would use it in real life too.
Ginny is a decent enough nn for it.
2. I like
Octavia too. I would also use it in real life but don't like it as much as
Ginevra and prefer the masculine
Octavius to the feminine
OCtavia.
3. Tave or Tavy seem to be the most obvious nns to me, other than Oct which doesn't sound pleasant at all.
“It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.”
~Mark Twain