I'm almost certain that if something like this happened to any fediverse instance - that a local police enforcement would contact the admin and asked for user's data, which they are required by law to provide or they would go to jail/get a hefty fine and possibly a criminal record, they would do that too. That's also why E2E is required, to prevent such problems for instance admins - but then again, there's really nothing you can do against local law, and if it requires that you have to be able to cooperate, well... Then there's not much the admin can do, without putting himself in a real risk of prosecution, because he is breaking the law by have E2E.

That's also a good reason to be careful when selecting your home instance, and making sure that you choose one in a country that has all right laws in that regard.

Of course, that's assuming the police makes contact. I don't suppose that the admins would be searching through the DMs of people to snitch on them. And if Meta is doing that preemtively and is actively snitching on people - that's downright evil.