SOLVED! There is a cutscene after which you will be teleported to the High Wall of Lothric. This cutscene sets the flag that allows the key-bearing corpse to spawn. Here's how I think it goes down: Not only does it check for three Lords of Cinder having been defeated, but also checks for quest-related scripts to have fired before triggering the above cutscene. Per /u/TrollOfGod's suggestion, go to each of the bonfires of the three Lords you have defeated and check their boss rooms thoroughly, for people to talk to and items to loot. Once you get teleported to HWoL after a cutscene, you'll know you've set the correct flag.

Original post below:

Here's my current game state:

• J3
• SL200
• ---
• All thrones filled except the central one
• Early ---: kill, but otherwise proceeded as normal
• ---'s questline completed
• ---'s questline complete up to this point.

When I killed the ---, it got me as well. I was dead but I got its Soul, and could use its bonfire. Heading into the Grand Archives the corpse that was supposed to be there didn't spawn. Am I screwed?