OP, I think what is difficult here is that whether you say you have or not, your story has changed quite a bit, making it quite difficult to follow. You say you could hear and see everything, but you didn't know what really went on, which shows that you didn't.
FWIW I think it's unfair parents calling your son a little shit. He was pratting around with a trolley, which kids do. They make mistakes, they are kids. And those saying "well my DC would never prat around with a trolley at 9" - no, maybe not, but I bet they prat around with something else and make other types of mistakes.
I pick up on what you are saying that she told your son off before he was rude. Again, we don't know the full reason. Was she just being an old bag sticking her ore in? Did he do something in those few minutes you were getting the car and deserve a telling off? Did she not realise that you'd told him to stop or not hear you and thought she was genuinely being helpful because thought he'd been left to his own devices? It's hard to tell, and in reality, the only person who does know is your son.
However, you did miss a good learning opportunity there. Your son was rude to the woman, which even he admits to doing it. Nobody deserves that. I'm not saying that makes your son a bad person, but that was bad behaviour and you were wrong to dismiss it. What does concern me is that he is open about mocking the woman to you, which means he obviously is not scared of the repercussions - because it is evident as he has tried and tested the water and this is something you obviously let slide. This is something I see when observing weak teachers - they let the cheekiness slide because they think they'll grow out of it, or stop on their own and whenever you tell them they need to clamp down on it because it will get worse, they shrug it off. And I'm telling you now OP, if you continue to let the cheekiness slide, it is going to get worse.