You've my sympathy for that awful experience.
I don't believe you're correct though with your statement that the brain has that much extra capacity. As I understand it a brain works by association. So when it receives signals which it associates with mortal danger, it will look for associated memories.
Basically you only think that you went through all that. Youll have a -memory- of thinking about everything you had done, but you didnt actually -think- about it. It is similar to doing an internet search and getting 10 million results. Even though you had that many results you will only really evaluate the highest rated results and in the end fully process one or two.
Additionally your brain plays a trick on you. When you recall the event it will bring up the memories while you "evaluate" them. But in fact that is the first time you really think about them. You're following the links that were created at the event, and the deep links, all the while thinking that you've thought about it before. Your brain does things like that all the time. A good example is your vision. Your eyes don't actually see everything youre `seeing. They only see a bit and your brain just extrapolates the rest, fooling you into thinking youre seeing the whole picture.
I hope that makes as much sense typed out as it does in my head. ;)
All the best