If a government recognised that an adversery of theirs had a lot of bitcoin reserves, couldn't they just 'crash' the bitcoin network by spinning up enough full nodes to gain 51% of the network (not a hard thing to do for a big government)?
Technically most of us citizens are this adversary due to fiat economics.
How would any particular node that blindly connects to one of these 51%'ers know not to interact with it?
Likely because of what you said
they could just stop receiving/validating/appending new blocks
This causes people to switch node endpoints if they notice such a thing.
How would a given node 'know' who to query for chain data?
They would do it as usual, using network consensus rules to verify the longest chain.
Btw, I understand the government would have NO economic interest in doing this.
It's not that they wouldn't have interest in doing it, it's that the current state of the network renders their attempts absolutely obsolete. 50% attacks on Bitcoin are highly unlikely for a variety of reasons:
the only relevant nodes in terms of establishing network consensus (about new blocks) would be nodes that miners submit their proof of work to which are generally considered "mining pools". These pools are actually chosen BECAUSE of how secure and decentralized they are, when any of these pools act out of the interest of the miners (ie ordinals filling a full block on luxor pool) it causes them to fall out of the top pool ranks ie miners go somewhere else. Not to mention since these pools are so big, having a central actor or even a hand full of them control the whole thing is highly unlikely, if they did anything wrong miners would notice within the hour max and switch to another pool. If they were in a proof of stake system they would be frozen from withdrawing from the attackers pool.
Considering this is the difference between pool maintainers making a good living and being bankrupt I would say the incentives are HIGH to keep it secure and decentralized.
I would really be entertained to see the counter arguments.