I can understand the problem really well. There was a time as a young teenager when I was having unsupervised visits with my manic-depressive mother who wasn't all that good at taking her meds. I never knew what to expect. Some visits were great, other times we'd get into a huge argument before even saying a proper hello to each other.
What was arranged for me is that an adult with a car, usually a family member, sometimes a parent of a good friend of mine, would always be on stand-by during my visitations. Any time that I felt uncomfortable and wanted to leave, I just had to call and someone sane would come and pick me up and take me home or to their place, no questions asked. It gave me a feeling of extra security, knowing that I wasn't "stuck" with my Mom for the whole visit but could easily get away at any time.
Your stepdaughter is also a young teenager now, around 13 or 14 I think? So perhaps a similar arrangement with a prepaid cell phone might work to avoid any such similar situation in the future?