Very nice.
I won't add too much seeing as how this is a bit of a laugh, but I suppose the mitigating factors would be that:
1) assuming people survive they would be acclimating to the background count. So the point where the background count is gone isn't F hours but meets an equilibrium somewhere short of that
2) the bomb creating a background count is one thing, but there are other forces that can counter that. The earth awakened with magic (... I know right), so it's lore that nuclear bombs don't work at their true capacity because mother earth just kinda says nope.
3) if we all had a dollar for every inconsistent or self contradicting element within this system we'd be rich enough to hire editors
So my only contribution would be that the environment would return to normal sooner than you'd think