Think of it this way: 35 ft is a little over ten yards. An athletic kid -- even the size of Brown -- could cover that distance in about 2 seconds. That's not a lot of time for a LEO to retreat to safety.
I agree with your observation that Brown could have covered 35 feet very quickly. To me, the most logical explanation would be that Wilson started firing when Brown was still at distance, with the last shot being fired was the one to the top of the head as Brown was falling forward.