The magnetic field (or lack thereof) isn't really relevant to atmospheric mass loss (IIRC Earth's magnetic field overall increases its atmospheric loss rate slightly). The lower gravity is the big problem.
Any impact big enough to break the Moon up would blow Earth's atmosphere off as an immediate byproduct of the collision, so we wouldn't really have to worry about "what happens next". XD
Charmingly, our "universal" healthcare doesn't cover prescription drugs. If it happens to you at the hospital, or your doctor's office, you're covered. Prescriptions, nope.
It can never be absolutely proven that the photon has zero rest mass, but physicists can by experiment place constraints on its mass. IIRC it must be less than 10^-17 eV. None of the follow-on consequences of a massive photon have ever been observed.
"Dark matter" is a perfectly appropriate name for it. Desiring the non-scientific to not get confused by science was a lost cause since before science was a thing.