That doesn’t work with distributed systems. No single country is able to shut the whole Internet down, for example.
(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
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.
Countries have this neat tool called violence that tends to override all security whenever humans are a linchpin