"First, both bombs were detonated more than 500 meters above street level so as to wreak maximum destruction (surrounding buildings would have blocked much of the force of ground-level explosions). "
I won’t call that wrong --although I think it probably the ground they were thinking about rather than the buildings (the Nagasaki damage was much smaller due to ground uneveness, and the Hiroshima buildings didn’t seem to be much of an obstacle) –
But the scientists (if not the military) already knew that a ground burst would be dirty, and would prevent occupation. One of the reasons for using air bursts in the tests was to prevent fallout and persistant contamination
“In 1945, Trinitite was marketed for use in jewelry, but it was later found to cause radiation burns if worn for extended periods of time.”
It is still considered a hazard. Most of the on-site material was buried and access is still limited to 2 public events a year with a strict no-collecting policy.
There was some testing of nukes underground (besides just weapons testing) for possible use in mining and engineering. But the long term, left over radiation in those cases ruled this out as practical.
Bikini Atoll shows that multiple tests of ground/ocean bursts leaves lasting problems.
Hiroshima and Nagasaki were sort of special given the knowledge and techniques of the day.