“Except the first point, which is that it was public property.”
You can be arrested for trespassing on public property. It happens all the time.
“The arrest was determined to have been invalid based on that alone.”
Perhaps, but that doesn’t mean the court was correct. If she was in an area she wasn’t supposed to be and refused to leave when asked, then it was a perfectly proper trespassing arrest, whether the courts agree or not. The courts judge facts but they don’t determine reality.
I know it does. It doesn't make what she did illegal. She was not preventing anyone from doing anything, she was not violent, she did not damage property, she wasn't there when it was closed, and it was open to anyone else who wanted to be there peaceably at the time. Bad arrest.