The difference with slashing is that forking is no longer "free" as each fork penalises members who only support the other fork. Let's consider your example in PoW and PoS with slashing. In PoW both chains would continue and miners can freely switch - their funds are also duplicated. In PoS those that support each chain will eventually be locked in, and penalised if the other chain "wins".

This doesn't prevent all (or effectively enough) stakers being coerced/forced by a powerful enough entity, but that's a social problem that no consensus algorithm can resolve - just as if effectively everyone was a bad actor.

Which governments, saying to whom? And how would they enforce it? What would be the consequences for those that didn't comply?

There are other proof-of-stake blockchains other than Ethereum, and for those it has indeed worked quite well in practice.