Nothing really says so, they’re just no stranger to working with/under him in the face of massive threats as seen in Legion.
When we did so back in Legion for the frost artifact, it was clear that while contained the local undead were certainly not controlled. ‘tis a dangerous thing to wander Arthas’ hallowed halls.
Implied to be stolen from the front lines, as they all made the ultimate sacrifice for their people.
Nothing, its just hard to show the player character while the player character is someone else. Though it’s safe to assume the usual happened: The title became decorative as the PC pursues their own interests.
Nothing says they don’t, and as far as we know they’re risen by the same spell so maintain the status quo.
A few major NPCs know it, and pretty much any DK in service to Acherus knows it. The average joseph and josephine are unaware.
It also generally seems to be so that they know there is a new one, not WHO it is.