A) the actual complexities of a city made out of separable bulkheads all watertight and self sufficient is far beyond anything humanity has ever built, and things 1/1000th as complicated (like the soviet moon rocket) exploded when we tried.

B) Small scale seaweed farms are not going to feed 200,000 people and if they could the people would be hugely nutrition deficient and anyway the ecological collapse caused by such an event might ensure the seaweed doesn't even grow, with the nutrients in the water being eaten by massive algal blooms or worse

C) Major industries like metal smelting, plastics manufacturing, oil refining, chip fabrication, etc do not function on small scales and eat enormous amounts of power which you would not get from floating solar

D) We're talking about a 6 month dust cloud so the seaweed doesnt grow and the solar panels barely make any power for 6 months...everyone dies.

Just move to Kathmandu above the water line instead. It's cheaper and far less likely to fail