I am interested in hearing more information on it. I mainly want to know if I can free lost souls and return them to the mortal world. As in, would it be plausible that if say Mannimarco was in there, could I unleash him into the mortal realm once more?

My lips are sealed. Sorry I can't say anything, nor can any other people who know about the Soul Cairn.

That's okay. Hope you're having fun though <3

Check out the wiki. That's how i sated my thirst for knowledge. Granted. Its not much but it was enough to get me through.
www.uesp.net/wiki/Skyrim:Soul_Cairn

Not long left

there's no much lore on the Soul cair, a bit from Tes: Battlespire, there is for example no deadric ruler, but it still have its own masters

I dont get it Who actually go to soul cairn when they die? Some go to sovngarde,But soul cairn? How do someone end up there?

The trapped souls go there.if you cast soul trap on someone and you kill him his soul will go to soul cairn

Now I feel like I'll be Evil if I trap souls, which makes my evil character even better

Really?! And here I was with my good character trying to free as much victims as people by emptying the soul gems ...