Really? That's fascinating. You do realize that microwaves can be blocked with a simple metal screen, right? The same sort of thing you see in the glass of your oven?The microwave signal does not detonate the explosive. The satellite provides a microwave signal that simply activates the warhead's trigger. The microwave signal acts on the trigger, not on the explosive.
Microwaves are radio waves, EM radiation, nothing more. And their effect on materials is a simple one. They cannot tell a circuit to "turn on." They can heat materials, little more.
The scientists who design nuclear weapons know that they must function reliably in a hellish environment of all sorts of radiation, heat, electromagnetic pulses, and must detonate ONLY when commanded. My guess is that they are immune to external radio waves.
It seems to me that you are joining a rapidly growing club of people who ascribe a technological level to the USA that is little short of magic. If we can detonate nuclear weapons with some intercontinental radio beam, why are we using propeller-driven drones to kill terrorists, when we can cook them in their sleep? Why are we spending billions on ABM technology?
This whole thing sounds like a scare-mongering chain email, little more.