Kithlan is essential, but USIP makes him unessential after he has served his purpose. He also has a Gravestone at the New Sheoth graveyard, but it is named after his old name: Alason's Tombstone.
The UESP says
Kithlan's original name was intended to be Alason. A variable in the quest script for the Ritual of Dementia, set for when Player persuades Kithlan to help, is actually called AlasonTold. There is also a tombstone in the New Sheoth Graveyard that is for an NPC named Alason, but since it is always disabled throughout the game it can never be viewed. The inscription reads: Chappy didn't love me enough to write an epitaph.
If i understand correctly, since Kithlan can now die, the Gravestone will get enabled. If this is the case, 00050874 should be named correctly.