The strongest hero.

I think that Agrail didn't bow to anyone, but when he "joined the service," he pursued his own goals.

He entered the service of the Overlord to escape his enemies. Agrail was a demon worshiper; he had no choice but to submit to Kha-Beleth. If Agrail hadn't been afraid of the Overlord, he would have thwarted all his plans regarding Isabel long ago.

There is no canonical information about Saretha; in 2 out of 4 endings, Saretha chooses her mother's side.

Still, she did submit in one case.

Did he deliberately surrender?! And disgrace himself before all of Shi'o? And release the rulers of the 4 main powers and his mortal enemies from his lair?!

Disgrace himself? Before whom? Before his miserable subjects? He needed to appease Ashan, to make him understand that everything was over, the demons were defeated, the war was over, etc. And while this was happening, he would let "his people" infiltrate and destroy the enemies from within. As he said, "I will win the war."

The huge demonic army was defeated by an equally huge combined force.

How, then, do you explain the incredibly large army that Biara had in the Claw? And what about the other demonic armies scattered throughout Ashan? It seems that such armies could not recover in numbers, even to half their original strength, in just six months. Kha-Beleth had been gathering this army for over 20 years. He could not have lost it in a single day.

The best agent certainly caused a lot of trouble, but one of the results of his activities was the destruction of almost all demonic sects and cults in Ashan, so it is very unlikely that Kha-Beleth would have surrendered in battle for the dubious opportunity to put Biara on the throne of the Empire for a short time.

Firstly, it was not Biara who destroyed the demonic sects; as far as I know, she only contributed to their development. She simply didn't anticipate that the undead could also think.
Secondly, the Overlord knew who he was sending to the throne, namely his best agent, and he trusted Biara, so for such an opportunity, he had to lull the enemies into a false sense of security.
Thirdly, by sending Biara, the Overlord also did not foresee the danger from Arantir. But this was not so important; Arantir would not have gone any further if it hadn't been for the orcs, who tied Alaric's hands. Kha-Beleth was counting on Biara, in the person of Isabel, to be "serious and for a long time."

and to unite Erathia and become the second lord of all of Erathia after Belketh – not everyone can do that, so I wouldn't underestimate Arantir.

Arantir is a fanatical fool; he became Lord not because of his extraordinary strength, but because of the weakness of the other necromancers. The one who is much stronger is Markel. Markel was the most famous necromancer, the strongest of them, and, of course, the most cunning. He was the only one who could raise thousands of skeletons and dozens of dragons, and at the same time legalize the existence of such an army, and where – in the Holy Empire! He killed the strongest mage of the Silver Cities, managed to imprison Godric in a dungeon, and waged a successful war on 4 fronts! And at the same time, he protected himself with powerful magic.