one thread is enough, you don't need to post four
However, welcome to the forums, I present you with your very own http://images.uesp.net/c/c4/Fishystick.jpg
Four? He only has three posts
But yeah, one thread will do, OP.
http://images.uesp.net/c/c4/Fishystick.jpg.
Well if Krosis isn't getting out of the coffin and the dragon isn't giving up its soul, you may need to either reload from an earlier save or wait for a few in-game days for Shearpoint to reset so you can try again.
Ah well if you've had it before and you've either misplaced it or sold it then you're going to have to go looking. There is only one Krosis mask in the game. Check the inventory of every follower you've had with you along with whatever shops you might have sold it to. I think because it's unique it'll stay in their inventory forever.
Dragons not giving up their souls is a common bug, unfortunately. As for Krosis' body, it despawns after some time, as all dead bodies do. Otherwise, the world would soon become follded with dead wolves and bandits.
Nope. Dragon Priest Masks, as well as other artifacts rewards, will disappear eventually after being sold.
Only required unique stuff will stay forever (like dragon claws, keys, notes, and more).
Plus a dead body with a required unique item will NOT despawn until the items are removed... A dead wolf near my house never despawn or respawn yet his "brothers" respawned about 6 times until I finally search the dead wolf and picked up the keys I dropped on him. (Which caused him to eventually back to the respawning cycle).
~Edit~
Was partial incorrect with the story. The wolf I mentioned did respawn but it still had all the items.
However, if you killed something which does NOT respawn then it will never despawn until you take the items back.