I'm asking about taxes for her, her name changing, and other legal stuff that would indicate we are married.
Ahh, so you are asking about this stuff in order to keep it quiet from the family?
As far as taxes go, I believe you can still file separately if you desire. Although, it's not like your family is going to be checking your tax returns.
I also found this website that covers all the aspects of a bride changing her name:
http://www.bridelaw.com
After a quick scan of the site, I don't see any time issues involved, so your wife-to-be could change her name after the "real" wedding.
The only timing issue I could find is to notify the Social Security Administration of the name change before she files a tax return with her new name. If she failed to do that her name wouldn't match her SSN, and the IRS would reject the return.
Practically, however, she could get probably do it all after the JOP marriage and just be careful around family.