I could not have the time to test this theory so I am asking to the community: if you were to soul trap an NPC that was meant to appear in Sovngarde (like Ulfric or Rikke) and send them (lore-wise) to the Soul Cairn (I know that the souls of killed NPCs do not appear in Soul Cairn) would the game still spawn their souls in Sovngarde or would you simply not see them when you get there?
Compliments to anyone who took their time to test this rather tedious theory before this was posted.
And thanks to anyone who tested it after.