Possible explanation for Imu's Amnesia
Event
In Elbaf, Imu, possessing Gunko, learned that Elbaf has no king thanks to Sommers, so he tried to make Dorry and Broggy the new kings of Elbaf. However, when Imu descended, he asked who is the King of Elbaf in which he got the same reply from Dorry and Broggy which is there is no King of Elbaf.
Explanation - Im using Lord of the Mysteries to explain this phenomenon so spoilers ahead
>!I think forgetting what happened when he merged with gunko while in his "hybrid" form is actually following a certain trope that I saw from a different light novel called lord of the mysteries (LOTM). In LOTM, a character who uses his clones to possess things (anything really) which similar to Imu. However, the character's true body is not knowledgable about what his clones experience so he has the clones write notes so the main body or a different clones know what happened. My guess is the same. My guess is that Imu sends out a clone to possess his subordinates body through a contract; however, he does not know what is truly going. Which is why Imu uses his subordinates to explain to him what is going on. !<
Remember this, Imu was in contact with the gorosei during the god valley incident so my guess is that Imu (true body) over heard the conversation as he was dealing with rocks.
This is just my hypothesis or explanation for Imu's amnesia.