Most of the science going into the Manhattan Project was experimental measurements and phenomenological models, not fundamental physics at the QM level. There were no usable quantum-mechanical models of nuclear physics at that point.
No; they didn't really need it.
It’s just a coincidence that they employed so many experts in quantum mechanics to do those nothing-to-do-with-quantum-mechanics experimental measurements.
Coincidence. If you wanted to do something with elementary particles, you couldn't possibly ignore quantum physics.