To be able to play with her, she will need her own account.
All your progress has been saved.
All new content will be available.
All old content is still available.
As for the last one, thats up to interpretation. It's still STO, but cryptic has added quite a bit.
Hopefully this answered your questions. If not, feel free to call me a moron and ask someone else
(Preferably without the moron)