0) yes they do, in extremis half a dozen HAEMP would basically take out the whole internet

(b) "seal the network in their borders" is much easier than that, lots of networks basically act like that already due to either voluntary compliance with local laws, or due to direct government interference with the networks

iii. the miners aren't really all that distributed, they group together for the same reason everyone else groups together instead of being free-range anarchists

[δ] all you need to do to shut down a currency within a country is arrest people using it, which is very easy and has a long history

They don’t need to shutdown for everyone , they need to shutdown only for their country .

Countries do this all the time , just yesterday we were talking about internet shutdown during elections and exams , then there are restricted countries like say North Korea.

The US is certainly capable of shutting down the internet with their control of some critical pieces of infrastructure like ICANN. They haven’t ever used it because levers like those are ones you only get to use once before other countries build an alternative, like when China shut down rare earth exports to japan years ago so countries like the US started up mines that were previously not worth it