You have met your resignation requirements if you have figured 12 work days into your leaving date. When she comes back from vacation, she may very well walk you out at that time. That's fine, you met the requirements you needed to. If she walks you out before you have worked the 12 days, you have not been fired, you resigned. She can spin it any way she wants, but you followed the rules, so you are in the right. You can not be fired, after you have resigned. Additionally, after you have worked the 12th shift, even if you are still on the schedule, she can't make you work more.
If you are denied your PTO etc, then you will need to follow up on that through the labor dept or lawyer.
Good luck in your next endeavor, you have done the right thing leaving if you were working in a hostile environment. A lot of us have made the mistake of staying, when we should have left. I hope you'll let us know how it turns out. Try and remain calm and poised no matter what she throws at you.