Both scenario A and scenario B have significant flaws in their "analysis":
A: Just about any country would be able to determine if an interceptor took down the missile/payload. It's not like it could be done and then denied. China tried that with their direct ascent ASAT a while back...didn't work too well for them.
B: A high-altitude EMP wouldn't take out the US's sensors...it would seriously disrupt civilian life, though. Our sensors, comms, silos, etc are hardened against just such an attack. Since it would be viewed as a nuclear attack against the US, I'm guessing conventional force strainage wouldn't really matter WRT our response to nK.
“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”
― Albert Einstein
Doing the popular thing is not always right. Doing the right thing is not always popular