a) I have cleared hag's end twice before, once for fun and another time for a radiant quest.

I found that no swtich wall before the quest, clipped back, looked around, saw it was just a chest, and left it alone. never saw body. always wondered about that chest.
c) now a switch appears. I use it. i see the chest.
I did a reload because if I travel to hag's end I have to kill all the forsworn nearby to fast travel. so when I could not find the body, I reloaded for later.
i promise I am in right place. you go into the place, kill 4-5 witches and a hargraven. you open a door that looks like it is just a crap boulder room but go up the stairs on the right. at the top of the stairs is a throne room with 2 witches a hargraven, and a level that lowers a bridge. on the left of the wall behind the throne is a switch (that was not there before).....using the switch shows a chest. walking to the chest autocompletes the quest, as the non existent corpse should be right there.