The Lich King was to get a Paladin for the Archon’s Sigil as Devos joining wasn’t part of the Plan.
There’s a reason Arthas wanted to convert either Tirion or Bolvar to Undeath.
Once Arthas died and Bolvar became the Lich King the Jailer switched to getting the Denizens of Azeroth to kill Argus to shut down the Arbiter and empower Sylvanas so that she could beat Bolvar, shatter the Helm of Domination and get the perfect Champion for his Bastion infiltration attempt.
The only hiccups to the plan were N’Zoth’s schemes to kill Azeroth before Zovaal could sacrifice her with his Heart of Azeroth and Sargeras’s plan with Xylem!
Khadgar being uncorruptable made it easier to confirm that neither N’Zoth nor Sargeras would be able to corrupt Dalaran’s leader into letting the Legion or Void win. Both took the hint and focused their efforts on other more useless targets or simple brute force.
The Void dropped all subtlety by making it obvious that Xavius was involved as did the Legion who after having their assassins in the Throne Rooms uncovered went all out in invading the Broken Shore.
Denathrius and Zovaal of course continued their subtle manipulations by replacing Matthias Shaw and Archmage Vargoth with Nathrezim.
The Legion having the Nathrezim in their ranks naturally noticed Vargoth’s actions and realized that they could continue with subtlety using other Archmagi and had Archmage Xylem whom they before hand corrupted before Cataclysm steal the Focusing Iris for their plans(namely to transfer all of Azeroth’s power to the Legion’s Void Revenant) only to fail.
The Void didn’t have time to react as the Nightmare was actively being invaded by the time news of the Nathrezim’s infiltration of the Kirin Tor was discovered!
https://wowpedia.fandom.com/wiki/Closing_the_Eye confirms that the Legion was behind the Void Revenant possessing Xylem and that they wanted the Void Revenant to claim all of Azeroth’s Arcane Magic killing her!
The Nathrezim’s infiltrations were for the sake of infiltration while the Legion’s infiltrations were for the sake of getting a Void Titan since Sargeras no longer could possess Illidan to open the way with the Sargerite Keystone! The fact that Sargeras decided to create a Void Titan loyal to himself using a Void Revenant loyal to himself as the base shows cunning!
If Sargeras had succeeded with Xylem he would have done what the Void Lords dreamed of doing but on his own terms! The Void Lords would have screamed in terror as Sargeras’s Void Titan brought the Void and everything else completely under his control!
Sargeras noticing Vargoth and putting his agent Xylem into play almost derailed Zovaal’s scheme with Azeroth!
The Void only started moving after Azshara’s power play in Stormsong Valley was foiled and the Void was solely banking on us defeating Azshara, releasing N’Zoth and corrupting us(first using a fake Wrathion with the backup being N’Zoth straight up corrupting us with his Void Magic) while sending it’s Sun Prophet to infiltrate the Amathet(who noticed while preparing a plan to kill everyone except Azeroth with Reply Code Omega).
The Void after just scrapping the idea of subtlety due to seeing them fail with Khadgar couldn’t create new plans for subtlety once they realized it still had a place. Sargeras due to being a being of Order on the other hand was more than capable of quickly coming up with subtle schemes!