Note: this comment is just about what is technically possible, not what should actually be done.
Actually those issues could be addressed by using a sufficiently large threshold system to protect each message's decryption key, so that in order to decrypt a message a large number of people have to agree that it is justified.
You could make it for example so that a message can only be decrypted if it is approved by judges in 3 different federal circuits, with at least 2 judges in each of those 3 circuits approving, and also approved by at least 2 outside civil rights groups chosen from {ACLU, EFF, Amnesty International}, and INTERPOL.
If that's not good enough we could also make it so that for each message the threshold system includes 5000 random citizens with smartphones and at least 3000 of them must agree.
Because network effects drive what chat apps people use. If people they want to talk to don't use that obscure app then they probably won't use it either.
Actually those issues could be addressed by using a sufficiently large threshold system to protect each message's decryption key, so that in order to decrypt a message a large number of people have to agree that it is justified.
You could make it for example so that a message can only be decrypted if it is approved by judges in 3 different federal circuits, with at least 2 judges in each of those 3 circuits approving, and also approved by at least 2 outside civil rights groups chosen from {ACLU, EFF, Amnesty International}, and INTERPOL.
If that's not good enough we could also make it so that for each message the threshold system includes 5000 random citizens with smartphones and at least 3000 of them must agree.