To make things easier, let's suppose our Activist is named Alice, and our Cop is named Charlie.
> The impersonator, by definition isn't known. Only the profile being impersonated is.
Charlie isn't able to impersonate Alice's profile because that's encrypted and Charlie doesn't have the key. Charlie can make a fresh profile, but it's not Alice's, although of course Charlie is able to use a stock photo of Alice (if he has one) and name it "Alice". Let's label this Alice2.
> If an activist is part of a private group, and a cop impersonates the activist, they can get all the new messages without raising any doubt.
Although Alice may be part of a group, Charlie isn't and Alice2 isn't either. Messages sent to Alice (as a group member) are not received by Alice2, who wouldn't be able to decrypt them anyway. Charlie will need to (as Alice2) ask to have Alice2 admitted to the group. Perhaps he can pretend Alice dropped her phone and bought a new one. Depending on what sort of "activist group" this is and the threat level, this may be quite easy or involve an in-person meeting which will be difficult to fake.
Of course since apparently you're imagining Alice is actually still around, she presumably tells people Alice2 is an imposter and it's likely this goes very badly for Charlie even if it's not an in-person meeting.
> In a lively conversation, the cop can even send messages and expect the activist not to see them. Disappearing messages will help here.
If Alice isn't knocked off the network, none of this "impersonation" works. You seem to be imagining Signal is some toy message board system where you can log in as Alice - just need an SMS, but it's nothing like that, that's why subpoenas don't bring anything meaningful back when they ask Signal about phone numbers, Signal doesn't know anything.
> The impersonator, by definition isn't known. Only the profile being impersonated is.
Charlie isn't able to impersonate Alice's profile because that's encrypted and Charlie doesn't have the key. Charlie can make a fresh profile, but it's not Alice's, although of course Charlie is able to use a stock photo of Alice (if he has one) and name it "Alice". Let's label this Alice2.
> If an activist is part of a private group, and a cop impersonates the activist, they can get all the new messages without raising any doubt.
Although Alice may be part of a group, Charlie isn't and Alice2 isn't either. Messages sent to Alice (as a group member) are not received by Alice2, who wouldn't be able to decrypt them anyway. Charlie will need to (as Alice2) ask to have Alice2 admitted to the group. Perhaps he can pretend Alice dropped her phone and bought a new one. Depending on what sort of "activist group" this is and the threat level, this may be quite easy or involve an in-person meeting which will be difficult to fake.
Of course since apparently you're imagining Alice is actually still around, she presumably tells people Alice2 is an imposter and it's likely this goes very badly for Charlie even if it's not an in-person meeting.
> In a lively conversation, the cop can even send messages and expect the activist not to see them. Disappearing messages will help here.
If Alice isn't knocked off the network, none of this "impersonation" works. You seem to be imagining Signal is some toy message board system where you can log in as Alice - just need an SMS, but it's nothing like that, that's why subpoenas don't bring anything meaningful back when they ask Signal about phone numbers, Signal doesn't know anything.