Wouldn't Landrieu need to get more votes in Orleans than registered voters in order to get 49.99% and lose every other county?
Theoretically no, but that doesn't really mean anything, theoretically she could've won Orleans by her actual margin and lost every other parish by a single vote and thus won with just that.
In terms of the numbers needed to do so....it's really not possible to calculate since it's just isn't plausible as a scenario, period. It's like asking to draw what an entire EC map would look like if the Republican won New York but the Democrat won Georgia. Or how a Democrat would lose Cook County and still win a statewide election in Illinois.