And I'm pretty sure Mami inherited the apartment from her parents, who were apparently quite wealthy.
I would accept that explanation if her apartment was the only example, but then we've got Homura's apartment, Madoka's house with the huge empty fields around it, the rows of trees and decorative stream leading up to the school... living space doesn't seem to be in short supply in Mitakihara.
All of the reading I have done on Downfall suggests one thing: that it would have resulted in the total and utter destruction of Japan as a country and a culture.
Would MacArthur still have been in charge of the occupation? Because he's largely responsible for the Japanese government being kept relatively intact after the war. (And for letting a few thousand war criminals off the hook.) It was an unconditional surrender, and he
could have reorganized the whole country, but he thought it would be easier if he didn't and blamed the war on the actions of a few bad apples.
Germany's government totally collapsed and was rebuilt from the ground up during their occupation by the Allies, with the eastern portions being absolutely ravaged by the Soviets, and it hasn't stopped them from retaining their own culture and returning to be an independent nation within a few years (except for the part the Soviets kept). Trying to keep any of the defeated nations as territories would have been
incredibly unpopular, as it would have been an act of sheer hypocrisy to do so after framing the war as being to defend freedom and democracy from conquest by fascism.
Alternately, the Navy didn't like Downfall, and wanted to just maintain the blockade until Japan had to surrender for lack of resources. Given how bad the starvation was in Japan by the war's end, that might have killed even more people, but left the system of government intact.
You forgot? We got Sayaclones now.
The concern was more about the conflict of interest that might arise if Sayaka's duties ever required her to keep Kyouko in line.