On that basis, yes, you could also take out the Russian or Chinese grids this way.

North Korea could also perform such an attack.

I assume the UK can too; although I don't know the maximum altitude of a Trident SLBM, the maximum range suggests it could well be great enough for this mode of attack.

Non-nuclear EMPs do exist, but you should probably ignore them at this point unless you're actively working in that field, given all the easier solutions in EW including simple jamming.

The entire electrical grid of the US is destroyed you no longer have the means to provide clean drinking water and food to the nation. You would have immediate and widespread economic collapse. Even the military would be crippled in there ability to help since, per the article, 99% of their power comes from the consumer grid.

You can lose power for a few days and be fine because you are still supported by a massive electrical infrastructure. But a destroyed grid would be catastrophic.

Exactly how many people you expect to die will depend on your assumptions. Also on whether it occurs due to a CME (can expect humanitarian aid from the other side of the planet) or military action (other countries probably have more pressing concerns in this scenario).

What would your plan be to keep you and your family alive? Would you be able to protect your resources from others who also want to keep their own families alive?