Unless they compel people at gunpoint (which prevents the government from bringing a case), they will probably not have much luck with this. As soon as a user sets up a passcode or other lock on their phone, it is beyond the ability of even most parts of the US government to look inside.

It's much more likely that the government convinces one member of the group chat to turn on the other members and give up their phone numbers.

It's even easier than that. They're simply asking on neighborhood Facebook (and other services too, I assume) groups to be added to mutual aid Signal groups and hoping that somebody will add them without bothering to vet them first.

I think disappearing messages only works if you activate it on your local device. And if the man compromises someone without everyone else knowing, they get all messages after that.

But yes... it does limit what can be read. My point is it's not perfect.

compel that person to give unlocked phone

Celebrite or just JTAG over bluetooth or USB. It's always been a thing but legally they are not supposed to use it. Of course laws after the NSA debacle are always followed. Pinky promise.