I am torn here because this behaviour is very like what I often had when on playdates with DS when he was 2-4 years old (he got a diagnosis of ADHD and ASD when he was 6). But I would have removed DS from this situation well before the end of the playdate described as I'd have known he would struggle to control himself
It was awful frankly but what I started to do around when he was 3 years old was to give one warning for any aggressive or very defiant behaviour (my standards were pretty low to give him a chance - so no physically hurting and no ignoring if the instruction was based on something to keep people safe. but most other things I let slide)
If we had a second incident we left immediately, from wherever we were. This was not too difficult to implement as I didn't have any other children to take into account.
DS hated it and would often scream and scream as we left. I also had parents try and stop me leaving on occasion saying I was overreacting (usually the same ones who had very well behaved children , were not used to having to physically removed a screaming and kicking, hitting child from a situation and had not been party to my first warning to DS)
In addition I tried to avoid places that I thought DS would struggle with which included places with lots of other children playing together in an unstructured way .
Over time (it did take about a year), DS stopped hurting people when we were with him . This was a combination of us avoiding places that we knew would trigger him and us understanding when we needed to leave if we had made a mistake (before he got too upset) and also DS learning to behave better .
In the scenario described and in the other parent's shoes, I would have ignored the lying on the seats and given up my seat as well. However I'd have taken this as a sign that DS was reaching the limit of his tolerance for this part of the playdate so would have encouraged us to move outside
I would have given warning to my DS about the sand throwing as that could lead to getting sand in the eyes and counted as physically hurting in my book . I'd have told him that if he threw sand again we would have to play elsewhere as I couldn't allow him to potentially hurt other people's eyes.
I also would have got in the sand pit with him and OP's son and made sure to model the right way to play with sand. I'd have looked closely for signs he would do this again and try to head off that behaviour. If I missed the signs and he did it again, I'd have insisted we leave the sand pit and play something else. Sometimes this would be ok but often he would have protested very very loudly about this. I'd have then told him his choice was playing on something else or going home. This would usually work as he knew I most definitely would take home home otherwise.
The problem would be of course that the sand throwing was solved in the short term, I knew that he might throw something else later (He often threw things at this age, just randomly and out of the blue).
I wouldn't have cared about the going off on a scooter in the play ground as I knew he wouldn't have gone that far . However when he started throwing gravel, I'd have warned him if he continued to throw gravel we would go home as by now, I'd know he was reaching the end of his coping methods.
I'd likely would have had to take him home shortly after that anyway , as even if he did stop throwing the gravel (which he most likely would have done for a few minutes) . He almost certainly would have done it again a few minutes later (just because he'd forgotten what I had said) .