Using a server means the problem isn't event the same problem.
The General's Problem (as presented in the comments) is trivially solvable by multiple messengers WHO COME BACK to report the message delivered. Wherein, the messenger is an encrypted messenger and the general has the key (knowing which messengers were sent out).
In an ephemeral message scenario, you have to rely on the same idea, using an encrypted log of messages that both generals have access to (since they are both "on the same team", there is already coordination).
This philosophical exercise isn't practical, from the setup provided. I don't see the point in trying to solve for it without adding a ton of additional constraints, by which you can reach an optimal or no-solution.
The General's Problem (as presented in the comments) is trivially solvable by multiple messengers WHO COME BACK to report the message delivered. Wherein, the messenger is an encrypted messenger and the general has the key (knowing which messengers were sent out).
In an ephemeral message scenario, you have to rely on the same idea, using an encrypted log of messages that both generals have access to (since they are both "on the same team", there is already coordination).
This philosophical exercise isn't practical, from the setup provided. I don't see the point in trying to solve for it without adding a ton of additional constraints, by which you can reach an optimal or no-solution.