Putting his hands up in some fashion would, by itself, typically be seen as surrendering. But if hands are up -- or out -- while advancing on the officer, IMO that's not surrendering. There are enough witnesses saying he moved toward OW that I believe he moved toward OW.
Also, the evidence cones farther away out beyond MB's body .... they're marking something, which means something happened out there farther away, which means MB moved back toward OW.
As far as the "short span of time that this happened in, which makes those decisions difficult if you're OW." That's exactly what SCOTUS addressed in Graham v Connor. "it must be judged from the perspective of a reasonable officer at the scene—and its calculus must embody the fact that police officers are often forced to make split-second decisions about the amount of force necessary in a particular situation." (Source:
http://en.wikipedia.org/wiki/Deadly_force -- but you'll be able to source that decision in many other places as well.) Very clearly, the benefit of the doubt in a split-second decision like that must go to the officer. He was the only one there at the time who had to make an instantaneous decision to fire or not fire, and the stakes were high regardless of which decision he made. At least he's alive now, to face the death threats and the accusations of murder, and not dead, which he would very possibly be if he had made the decision not to fire. In that case, he would be just another cop killed in the line of duty, a statistic with no national media interest to mark his death, ignored by Eric Holder, and unimportant to the activists and instigators.