Kha-Belekh deliberately surrendered.

Deliberately surrendered!? And disgraced himself throughout Shio? And released the rulers of the four major powers of Ashan and his mortal enemies from his lair?!

I also find it hard to believe that the Overlord surrendered, but his line at the end of mission 6 of campaign 6, "You have won the battle, but you will lose the war," doesn't sound like suffering from defeat. However, I wouldn't say that the Overlord put up a small army against the four.

But the fact that Kha-Belekh didn't see through him reflects poorly on the Overlord.

He did see through him. Campaign 2, mission 1, the Overlord accuses Agral of betrayal and orders Erazeal to kill him.

Zeheer is still young.

Nonsense, he couldn't even defeat a young, inexperienced orc mage.

Zeheer and Gotai are, yes, young. But Zeheer is the First in the Circle, the most powerful mage of the Silver Cities, and Gotai did the same thing as Arantir: he united the disparate orc tribes (just as he united the undead). Yes, he doesn't possess magic, but the fact that he marched an army through the Griffin Empire speaks to his talent as a commander.

If "seriously and for a long time" means a year at most, then his plan succeeded. It is simply impossible to count on anything longer - either Biara will simply be exposed, since the real Isabel appears in different corners of the Empire, or her "reforms" will simply cease to be tolerated, including by the army, and they will decide to put Frida/Duncan/some duke on the throne.

Frida is the daughter of Godric, who was Isabel's bodyguard. The army would hardly run and fall to put her on the throne. Duncan is the Duke of Unicorn, one of many (12, I think) dukes. The Empire's army is the united army of the dukes, so if it rebelled, a civil war would begin, from which Biara would most likely emerge victorious.

Trusted!? Demons are not particularly trusting, and the Overlord even less so, especially after Agral's betrayal.

I agree with SAF here. But it is quite possible that the Overlord already had ways to influence Biara. And she doesn't seem to be a very ambitious demon, so as to betray Kha-Belekh.