How difficult (expensive, realistic, etc) would it be for the US gubmint to buy up enough mining power to hijack the network, change the rules, and send it all to zero?
An S19 Pro costs about $11k and has a hash rate of 140 TH/s. For a 51% attack, at least 200 million TH/s is needed, so about 1.4 million S19s, with a cost of about $16 billion. There is also the cost of the supporting infrastructure, labor and electricity.
As far as your concern about changing the rules -- it is not possible. Changing the rules would simply cause a fork, and I think most people would probably ignore it.
The most likely attack would be a denial of service. The government would prevent other miners from adding blocks and publish empty blocks for as long as needed in order to kill the network. In the end it would be expensive, but it is probably doable if there is sufficient political will behind it.