Well, remember China would probably be very, very pissed, due to the whole "Dropping a Nuke on their Doorstep." thing.
The EMP blast depends significantly on the altitude of the detonation, IIRC.
Yes. In fact, it ranges from "The enitirety of the North American Continent" to 10 miles.
It depends entirely on a huge number of factors, some completely out of our control (Such as the magnetic field, which amplifies shit in weird ways. It's Proven a EMP on the US would cause alot of damage beyond it's borders, especially in Canada). However, is mostly a either large range or short range thing, a Ground burst would have a range of 10 miles, but a low altitude one is almost unpredictable. And, EMP is not a "End of days Scenario", nor a "Shrug it off" sort of thing. We don't know. Last time a EMP occurred over a Civilian territory, It affected 50s or 60s era Hawaii, and shut off street lamps. We don't know what it would and won't affect now. It might not even bother most of North Korea, after all they aren't very advanced. On the other hand, non of their Computers are protected.
It is too unpredictable to pretend we can do it surgically. Most EMP studies were based around the idea of a Entire massive country, the US, the USSR, the PRC, not at all about limiting the effects. All I know about that is the more shields a bomb has (and therefore, the stronger the bomb needs to be, yield has little to do with EMP) the weaker the Pulse.