The only way i'd believe for a second this really happened is if...
a quick google search shows that anywhere between 55 and 75 camaras are rolling at the academy awards at any one time. even 360 degree camaras, one of these camaras would have caught what happened from Will going from sheer laughter to anger in the space of less than 5 seconds, i want to see what made him change so radically. the interaction between him and jada.
I think the camara people and directors made a mistake or Will did, he wasn't supposed to be filmed laughing like that, the joke was supposed to be told, then appears will, he slaps chris, sits down and then starts shouting what he did.
They can't show Will going from laughing his ass off to anger in less than 5 seconds, because if you weren't born yesterday then you'd know that it's not realistic.
When that video comes to light, then maybe......until then it's a staged hollywood bit. especially when we see him laughing before and after the slap.