If the consolidated city-parish of New Orleans had the density of Amsterdam, the population would be just over 4 million people, so 900,000 should be manageable. The long-term survival of the city would still be a problem.
What's the trade and tariff situation between La Floride and the Commonwealth? I ask because even though the Commonwealth's economy is more focused around the Great Lakes and the railroads than the U.S. economy ever was, an easy trade route to the Gulf could only strengthen them. If trade barriers weren't already down by 1950 or so, I can think of two things that could happen:
• The Commonwealth could offer its assistance in building an analog to the Old River Control Structure (the thing that keeps the Mississippi from changing course) in exchange for free trade along the Mississippi.
• At some point between 1980 and 2020, after a particularly bad flood, the Mississippi could drain into the Atchafalaya, turning New Orleans' portion of the river into a tidal channel, meaning however many people still live there would be drinking seawater every time the tide came in. At this point, the Commonwealth could make La Floride a much more ruthless offer: grant free trade along the river and an exclave along the Atchafalaya in exchange for help building aqueducts and desalination plants to keep New Orleans alive.