I don't think Max would have suddenly jumped to swing on the chandelier. He was reportedly a very intelligent, and athletic kid. I think he was plenty old enough and smart enough about gravity to prevent himself from attempting a swing on the chandelier.
The planking I could understand a little more, if he saw older sibs trying it, and if he thought he could easily stay balanced upon the rail. But the planking theory does not fit with the way his body supposedly was propelled over the top.
The scooter could not go that fast on a carpet, imo. And I see no way that it would have gone over the rail itself. Scooters are bottom heavy. It would have to have been going very fast for awhile to make it over the rail. imoo
The scenario of him running fast and missing the top step for some reason and then falling over the rail makes the most sense, I suppose.
In terms of the 'physical' outcome, it appears most likely he was thrown over the balcony, if you just take into account what LE described was the way the body fell.
But I cannot come up with any believable scenario in which anyone would have thrown that poor sweet boy over the railing. So I am going to assume that did not happen.