^ Yeah, that's an interesting scenario and problem.
But hey; can't the banks do all sorts of nefarious stuff too?
There would definitely be some issue with the block times and difficulty rate. We saw this soon after the Bitcoin Cash hard fork in late 2017, where hash power was swinging wildly between the two chains, and transactions on both were running normal, then at a crawl, then normal etc. Both nearly froze, and sensing this both came to a consensus.
It is possible that, say, a mining pool on one side of the planet is not hit, and has 50%+ of the hash rate. Are they going to attack Bitcoin? It seems very unlikely as their entire business, all its hardware, its treasury, its future, is in Bitcoin.
As for the hash rate: yes, blocks/mining would slow to a crawl until the difficulty is adjusted, either by reaching the block where difficulty is adjusted, or if
consensus was reached across enough nodes to adjust it via a software update. Consensus being the operative word here. I doubt given the extreme situation many if any nodes would indicate any objection.
Any nodes that don't agree can continue on with the "classic" version of the software, and wait until the block that adjust difficulty is mined, but with most miners opting for the software update (surely) that fork/chain would stop mining, and die. (or, they would accept the heaviest chain either way, not sure.)
So to answer your question "The hash difficulty is automatically updated every fortnight but can anyone step in and do it manually?": No, nobody can, but everyone/enough people can, if that makes sense.
A real world example very similar to this is the block size debate which saw some nodes agreeing to increase the block size and not implement SegWit, and others keeping the block size at 1MB and implementing SegWit. The two are incompatible. We now have Bitcoin Cash and Bitcoin.
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Bitcoin is anti-fragile. The more it is destroyed the stronger it becomes, sorta, in a weird way. It's best practise software design: fail often.
Not unlike the internet it is self-healing, and can survive being split in half etc. etc. No central point of failure.