The strongest hero.

Yes, he found a wonderful way out of the situation—he saved himself and became the right hand of the Overlord; there is nothing shameful or bad about that. However, the fact that Kha-Beleth didn't see through him reflects poorly on the Overlord.
That Agrail wasn't afraid of Kha-Beleth and even dared to be insolent to him is as clear as day, and there are many confirmations of this in the Campaigns and the scenario.
And why would he need to thwart his plans? He didn't even know that Isabel was supposed to give birth to a messiah; his only task was surveillance. And besides, Railag/Agrail ruined quite a few of Kha-Beleth's plans later on.

The fact that Agrail was seen through has already been written above.
Insolent? Well, yes, perhaps in the company of his gang in the 6th campaign. I don't recall any insolence in "Agrail's Trial." Quite the opposite, actually.
If Agrail is truly so ambitious, why wouldn't he thwart his master's plans?

Well, if that's the case, he didn't bow, he joined
And anyway, the prophecy of Sar-Shazzar says that the demon lords will bow before the Dark Messiah, so who bowed to whom is still a question.

Actually, Sareth was the one who bowed. He even calls him Lord.
And the prophecy says this: "And the horned lords shall bow before the one who is not yet born from the master of darkness."

Among these "pitiful subjects," there are quite a few demon lords like Agrail (i.e., ambitious), and could the thought of the Overlord's vulnerability have crept into their minds? He wouldn't risk the threat of a rebellion and a subsequent civil war just before the birth of the Messiah.

Where are these "ambitious demons"? They all died out hundreds of years ago.
Good lord, what rebellion? They couldn't even think of rebelling. Civil war? In Sheo? The Overlord was an ABSOLUTE monarch in his own country. All demons recognized the supremacy of Kha-Beleth and did not doubt their Overlord. As Biara says: "You doubted our Overlord? How could you?"

But so far he hasn't won anything, quite the opposite. Aside from the birth of Sareth, he has nothing to boast about yet.

He will win the war if Sareth switches to his side. Everything in its own time. And he can boast. At least about the fact that the Griffin Empire ceased to exist, the Griffin dynasty perished, and the new Unicorn Empire lies in ruins. Also, Irollan, the Silver Cities, Ygg-Shail, and Grimheim have been devastated. By the way, all these countries were beheaded. The results are not bad.

Did I say that the entire army was crushed? And could the entire demonic army be in one place? As I noted above, Kha-Beleth simply has a huge army, but all of Sheo couldn't be in one place at one moment. The quartet defeated all the troops the Overlord had at that moment, and not even the Demon Lord can summon the entire army instantly.

As far as I know, the Overlord always kept legions with him. To guard his castle, otherwise 20 years ago three kings managed to unexpectedly break into Sheo and besiege Ur-Hekal. And in general, in the description of Ur-Hekal it is stated: "a city... guarded by legions of sentinels." Yes, Kha-Beleth is simply obliged to keep countless guards near the tomb of his Lord!
The Overlord is the most powerful demon, and believe me, he can handle the summoning of his own troops in minutes, especially on his own territory, in his native element.

If "seriously and for a long time" means a year maximum, then his plan succeeded. It's simply impossible to count on more—either Biara will simply be exposed, since the real Isabel flickers in various corners of the Empire, or her "reforms" will simply stop being tolerated, including by the army, and they will decide to put Frida/Duncan/some duke on the throne.

However, the Red Church was supported by the majority of the Empire's population. Her reforms created the appearance of fighting the rebellion, and the people believed her. And the army—remember that zealot of the faith who spoke with Arantir. They brainwashed him well. And everyone else, for that matter. Moreover, half the army are hidden demons. And in general, the rebellion had already died down by that time; the Red Church crushed the rebels. If not for the interference of foreigners, the rebellion would have completely fizzled out. And the fact that they tried to put someone else on the throne is nonsense. They tried to crown Randall too, and look how that turned out. They tried with Andrias as well, and as a result, he "became" a demon-spawn. The propaganda in the Red Army was like that of the Bolsheviks.

Well yes, for over 100 years no one could unify Erish; probably all the necromancers were just very strong, and as soon as Arantir appeared in the light, everyone immediately weakened
Fighting through half the lands of the Empire and the Silver Cities is also not the most difficult task.
And the fact that he removed a complex spell from Isabel's spirit with a wave of his hand—a spell apparently cast by the Overlord himself—or smashed the Dark Messiah against the wall at the first meeting (as well as the not-so-weak vampire Giovanni), is that actually a sign of his weakness?

Markel unified Erish. If he hadn't been killed, Arantir wouldn't have even ventured out of his library.
Arantir passed through two duchies, one of which was occupied by orcs. Is that half? The mages had an internal strife, and before that, Markel had marched through their lands. Capturing ruins is indeed not a difficult task.
I don't think the spell was complex. Biara had already easily turned into Shadia and Beatrice before that. And the Dark Messiah was a human at that moment. When he became a demon, he smashed Arantir against the wall himself.