What would happen if the NerveGear was yanked from someone's head? In the anime it is said that if you try to remove the helmet it would destroy the brain - so what if you pull it off quick enough that it doesn't have a chance to?

Certainly, it's not impossible; and on that note, why did they not just use an EMP on the device to begin with, frying the circuits so there is no way for the battery to power the microwave transmitter?

Is there some law of physics I am missing here?
I can think of two reasons: 


Plot. 
They wouldn't want to risk someone's life for an experiment with EMP, who knows what kinds of backups does the NerveGear has, and EMP can be guarded against.


As for yanking, a computer would have a much faster reaction time than you.