As soon as there is another untrusted party in the encryption, an in particular a party with a "skeleton key" that can decrypt anybody's message, then your encrypted communications are merely one leak away from being decoded by everybody else.
https://www.vice.com/en/article/hackers-published-replicas-a...
The mathematics of encryption just doesn't play that way.
This is a political / social issue. A human has to decide.
>How does it look different from a court order signed by a judge held at gunpoint?
It doesn't. You could similarly hold developers at gunpoint to push malicous updates too or at users to give them their messages. Putting people at gunpoint is illegal.
>How does it look different from a court order signed by a dictator's minion?
Judges could have their own private key so it would be noticeable that the minions private key would not be trusted.
>How does it prevent someone from tricking a judge into signing an order to decrypt message 2421425241 instead of 2421475241?
It doesn't. This is solved by laws that make such an action illegal to do
>What is stopping the service from accepting invalid court orders?
They want to protect the privacy of their users so they should reject invalid court orders.
>What is stopping the service from just decrypting everything for convenience?
They would not have the keys LE has. So it could be setup such that LE's keys are required to decrypt something.
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.