Thucy, if you're enjoying Senegal then I doubt you have much to worry about with New Orleans. The standard of living in Louisiana in general is markedly lower than in other parts of the country, but you've already demonstrated that isn't an issue (NOLA is still going to trump Senegal). And that's really the only glaring negative I can think of. I think you'll love New Orleans; two of the things you've mentioned as being very important to you -- racial/ethnic diversity and new/interesting culture -- New Orleans has in spades. The food is great, people are lively and interesting, and although you don't have anything lined up yet like in Williamsburg, Louisiana in general hasn't struggled with unemployment like the rest of the country has.
You might not start out making $40k like you would at Williamsburg, but there's an important factor I mentioned earlier that probably hasn't been mentioned much: standard of living. You'll make less money here, sure, but things are cheaper as well. How much cheaper, dunno exactly, but you're not doubling your real income in Williamsburg vis-a-vis NOLA, it's a more moderate gap.
The other alternative I'd suggest is taking the job for a few years and either trying to see if you can relocate within the company to New Orleans or simply leaving the company and moving to NOLA and working for someone else. You'll get a LOT more mileage out of New Orleans if you live there younger, but that doesn't mean you can't simply defer for a few years for income's sake.
I definitely fall down on the side of move-to-NOLA though. There's much more economic opportunity here than people realize.