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?
All you have to calculate is for how 51% attack is possible, check the present bitcoin mining hashrates and see the total. So, an addition of 50% of the hashrate coming from the attackers will make 51% attack possible on bitcoin mining hashrate. Presently the bitcoin mining hashrates is at 213 EH/s. Miners from all over the world are contributing to the total bitcoin mining hashrates but that is not stable but either increase or decrease over time, but the hashrate is so high and strong to the extent no one can ever as of now think he wants to provide half of such total bitcoin mining hashrates. Even if such happens, it will be difficult to maintain which makes it not to worth wasting money, time and affort to carry out 51% attack on bitcoin blockchain.