Let's face it, no matter what he did, he couldn't win; if he had done what you're suggesting, he would have been accused of being a dictator, butting in on the business of the state of LA.
Agreed. He did weaken his hand with some public relations mistakes which played right into the hands of the democrats who were looking to pass on the blame.
His mistake was not being upfront with the American people about the proper role of FEMA and the federal government. Most of the American public are under the impression that FEMA is some sort of rescue service.
Rather than correcting that misconception, Bush enhanced it by calling for more federal powers during emergencies.
I do basically agree. Though, on this occasion he would have been vindicated when the levee broke.