A.G.
Hi Robyn-
I went through a similar situation with my daughter. She was fine when we were in our home but seemed to have accidents each time we went out. I wanted to give up the whole potty training because it was much more inconvenient to have the accidents nor did I want to be home bound all day.
For us, it seemed that she needed almost to mark her new territory. Even now, we almost always have to visit the bathroom when we are in a less then familiar location. It seems to give her some comfort.
What we did was: get a travel potty, encourage her to use the public ones when in the community and stay positive and encouraging. I would ask her if she wanted the pull up or the panties before we went out. We did not have the rule that you mention, I would let her try to let me know what she needed. I do also recall scaling back on activities in the community if the potty was going to be an issue (shorter trips out, etc)
Hope this helps! You are doing a great job!