Yup, got back when Warcraft was actually good.
You keep using indoctrinated when you mean inducted. You are inducted into an order, IE given a position within. You are indoctrinated into its beliefs, IE you are taught its beliefs.
Whos soul would you expect here the most? And yet there is no Sylvanas' soul in sight. Could this mean that her reward for working with the Jailer is getting back her shattered part of the soul? Maybe she isn't a broken husk, but herself, with a huge fivehead plan, like uniting the Shadowlands and give a fighting chance against 10.0 antagonists. God I hope this won't be dumb.
During this quest, while you're in the portal of souls, you are given a buff it is called Souls of Frostmourne - "Viewing many of the soul fragments that Frostmourne tore from its kills" therefore all the souls in the room are those that were trapped in Frostmourne by Arthas.
Arthas looks so stupid on Invincible. That mount has never not be gaudy.
So.. If those fragmented, blue crystals are fragmented souls... that leads me to believe that the Jailer had taken Sylvanas' fragment and put it into Kingsmourne, during it's forging.
I personally think by this point Arthas is still intact but has been twisted to be full team Jailer at this point. I'd like a raid or dungeon boss fight with him but lore wise Uther tries to save his student only to be injured. As Arthas tries to finish Uther off he is interrupted by Thrall then the two who had most defined Azeroth at that time but never met then face off for Uther's fate. Arthas vs. Thrall, Human and Orc, and in the end one falls for good while the other raises their weapon in victory and cries out "LOK'TAR!"
god i want that armor so bad