" but this certainly isn't the first time an incompetent local official has been allowed to hold off the Feds."
No, it certainly isn't the first time, and probably won't be the last.
The Chief Moose situation was very different. That was totally law enforcement. And maybe in that case, the mayor (the chief's boss) should have stepped in.
I think in a situation of a natural disaster, the local authorities are looking for help and advice, (this mayor certainly was), especially if they haven't been thru a similar disaster in the past. Sending an adviser to aid the mayor or governor, isn't taking all control out of the hands of local authority.
In the end, it would avoid any appearance of "dereliction of duty". The official would be advised on how to handle things and how to remain in charge. If anything, the mayor or governor would probably end up looking like a hero.
In this case, the Mayor was incompetent - a political novice who probably had no idea about what goes into governing a city - and the Governor, next in command, was also incompetent. She is another political idiot (teachers' union background, no command experience) and obviously is not up to the job.
Chief Moose was also a loser who had already been fired by one extremely liberal PD after gross failures there, and was hired because nobody expected anything serious to ever happen there and they wanted a black police chief. He kept warding off the FBI and other agencies, even state agencies, and was essentially able to project his political biases in such a way that he prevented a serious investigation. And this was known to many people.
We have a system of subsidiarity which, in principle, is a good thing. But when things go wrong, we don't really have a way for anybody at the next level to take over, and the ones who suffer, as usual, are the ordinary citizens. Heck, Moose got a book deal out of his incompetence, and I imagine that Nagin will, too.