That sounds about right. Water would follow the path of least resistance, which would take it right through flooded areas. The lake, the river, and storm surge all pooling (no pun intended) together in NO really could mess stuff up.
I stand by my filthy lake theory. The surge/river would top the levee a few feet at a time. Right next to the levee it would rush in, but the city would slowly flood as the pumps could not keep up with the ocean. In New Orleans my guess is that the old Miss barely flows at all, maybe just a few knots, not a roaring mountain river. After the storm the surge receeds and the river lowers leaving water in New Orleans to the top of the levees. The pumps are down because they were inundated and the water trickles out the storm drains by gravity. At -7 feet, gravity would only empty New Orleans until the water levels equalized with the lake/river. The pumps would be needed to pump out the last 7 feet of water.I may be wrong but I see it differently. If the ocean floods into NO and the lake floods into the downtown area, there will be one huge torrent of water from the flooding Mississippi and lake. The river will continue to make it's way to the ocean via the flooded areas.