The notion is this: although taking over 50% of the Bitcoin hashrate would be a practical impossibility for any private actor or even a small country, China, as a country, has at times hosted over 50% of Bitcoin's hashrate, and it's thus plausible they could make this happen again if they wanted to.
It wasn't the Chinese government that were incharge of the 50% hashrate but it was individual miners located in China and they'll all have different beliefs so this attack isn't possible. We have core believers of Bitcoin that are miners in China and they won't want anything to happen to Bitcoin Blockchain so they won't be in support of the Idea if it gets proposal by the government. We aren't the only ones tired of the government centralized fiats system and wants
a decentralized currency to succeed so you can forget about this thought of yours, nothing is happening to the way Bitcoin was built to operate, if after so many hard forks, nothing has happened and the Bitcoin Blockchain hasn't been affected, there's nothing any government will do to take down Bitcoin and be successful. The Chinese government have probably thought about this and didn't see a way they can pull it off.
5. Have major governments e.g. the US and NATO countries ever (publicly) announced any contingency planning around this problem? Putting a $1.5T asset at risk is something that could rattle the entire world economy. This seems like something they would at least think through?
They first have to accept Bitcoin before they can start planning any counter measure if this event was to come to reality but since the governments don't care about Bitcoin they won't have plans for this in their books. The Chinese governments are attacking Bitcoin and other related services in their country, other governments are doing the same thing in their various countries so what we should be more concerned about is the government coming to an alliance and try to ban Bitcoin globally instead of having this thought of yours but I get it as it's a discussion that shouldn't be swept under the rug.