I don't know signal very well but when I have spoken to others about it they mention that the phone number is the only metadata they will have access to. This seems like a good example of that being enough metadata to be a big problem.
The steps to trouble: - identify who owns the number - compel that person to give unlocked phone - government can read messages of _all_ people in group chat not just that person Corollary: Disappearing messages severely limits what can be read
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.