There is NO central authority that can enforce evacuation or rules of re-population for metro New Orleans. Only the Louisiana State Government could coordinate responses, and they failed to do so. There is blame on everyone's part, but State laws and authorities are the only means of coordinating the whole. But could they have done so?

New Orleans City (and the mayor) lack authority outside of the legal and artificial City boundaries which cover only part of metro New Orleans and about 60% of the population. Cities such as Kenner (between the International Airport and central N. O.) have their own governments, police, etc. (and we see nasty situations such as their mayor's refusal to give shelter to Hispanics working in their community).
Cities on the south side of the River, such as Gretna, also have their own government, police, etc.- (it was Gretna that locked down the bridge from 'downtown' N. O. to their city, keeping refugees out).

The President of Jefferson Parish is not bound by New Orleans pronouncements or orders. Part of his Parish is part of central New Orleans, but Algiers, Gretna, etc., are traditionally considered his government's responsibility. A municipality such as Gretna, however, will tend to go its own way.
If the Parish President says the Parish is open for business, only the State Government can naysay his order. His "welcome to Jefferson Parish residents," linked by Q on the 17th, has not yet been withdrawn although the New Orleans mayor, because of the new hurricane Rita approaching the Gulf, has issued a delay for the part of the Parish within his city.

In other words, there is NO centralized authority over metro New Orleans.

Most of Louisiana's population is in the southern part of the State, hence it elects a considerable part of the State government's representatives. The State Governor and his officials will do nothing to upset the southern part of their constituency. They seldom can be convinced to take the bull by the horns.

Now we see the Federal Government is "advising" caution, but unless it enacts martial law, it cannot force local authorities to comply with their advice. Its evacuation 'orders' cannot be enforced.

We can only 'wait and see' if, in the future, the State can re-structure the lines of authority to cover the entire New Orleans metro population.