The US doesn't have small enough nuclear penetrators to even try hiding something anyway, they can use tactical warheads as bunker busters but they can't be meaningfully hidden. Meanwhile all these Iranian installations have been built with conventional bunker busters in mind so the existing American GBUs that must have been used may not have caused much real damage after all even if they double tapped the exact same spot.

Yes. Different blast pattern, different fallout pattern, different isotopes, different seismic signature, different sound wave pattern…

Yes. Only in fantasy land. As Logi above said, nuclear detonation is an extremely precise, controlled process that has very specific conditions to achieve successfully. Even an actual fission bomb only manages to consume a fraction of the radioactive material.

The only thing someone would achieve by denotating a conventional explosive near a reactor or nuclear stockpile is spreading highly radioactive dust around. That does not nor will ever look like uncontrolled nuclear fission, let alone a detonation from a thermonuclear warhead.