You’re right about the consequences for NORK; but, according to the EMP Commission, you’re very wrong about the number of EMPs required.
IMHO, no state will risk an EMP attack on the USA, for the reasons you mention. However, a non-state terrorist might try it. The most likely scenario being a launch from a freighter, within a few hundred miles of the coast. While the U.S.A. would probably glassify NORK, Iran, and possibly other possible conspirators (deployed nuclear subs would still be operational) — that might not suffice to deter non-state terrorists.
We put some pretty serious hardware in the upper atmosphere years back while testing our goodies. We could not take out anywhere near the size of the continental United States, we knocked out some lights and made a lot of stuff fuzzy on the radio, but we did not blast a 3500 miles wide diameter area.
I too used to freak at the idea of a single warhead floated up 300 miles would wipe us all out if it came up from the southern hemisphere. Fact is, it won’t. Do damage? Sure, knock out everything? nope. Beyond which, how does a non-state terrorist get a payload into orbit in the first place? They would have to work with a nation in order to do so.
It would be a lot easier and cheaper to just shoot out a lot of transformers on the 3 grids at the same time.