The Carrion King was a mortarch already, and at least as far as I understand mortarchs, that's kind of a permanent thing. I also doubt Nagash just wants to force Ushoran back into his prison. Nagash's punishments, whether purposefully or otherwise, often make the victims more useful tools for Nagash, and when this happens Nagash doesn't double down on further punishment, he puts the now useful tool to work instead. See Vhordrai, or Tamra, or Olynder, or most of the Nighthaunt units.
The CK's offense and punishment predated FEC as a faction. Mordants likely already existed, but Abhorrents afaik didn't exist until after the CK was freed by Sigmar and loosed into the wilds of shyish to spread his madness. In time that madness spread to other Soulblight vampires - whether new progeny of a maddened CK or the CK's madness spreading to other already existing vampires - and then spread from these new Abhorrents to the Mordant communities they encountered and came to control.
FEC remain a powerful but unreliable force for death. The Abhorrents can successfully control the Mordants, but the madness of the Abhorrents prevents Nagash & his mortarchs from using the Abhorrents to control the FEC generally. But if Nagash were to get his hands on the Carrion King, he may be able to use the CK to control the Abhorrents to then control the FEC generally, making them a reliable force hidden in the nooks and crannies of every mortal realm.
This, I imagine, is why Nagash is so interested in recapturing the Carrion King. It simply makes more sense than putting a lot of resources into finding the monster just to put him back in a box.