So I have no idea how to internet (read: upload a pic from my pc) since this is my first post, But! I may have found us an entrance to the fabled 12th piece!
If you go up on the ledge where that crate is for the whatever quest in Wolf's Hollow (don't hate me, I'm 107 hours in, I can't remember the name
) and then begin to proceed (faceing the crate) to your left, carefully around the hex-stones, you'll notice an opening in the stones that appears to lead behind the cliff face. You'll also notice what appears to be a larger opening a little higher up and slightly to your left. I am going to use the same cine tool the person that found piece 12 used and see if I can map out a route. But in the meantime, anyone that wants to try finding a way up there (I'm terrible at wall and cliff jumps) and see if you can find a way in, that's awesome. Best of luck everyone!
Edit: welp, that sucked. Definitely cannot get up there that way. I did see someone managed the mount-jump trick at the hex steps to the left of the entrance to the hollow, and managed to continue on to the Right side once up. Has anyone else managed this? I cannot, sadly, do that, I have to keep going around left, then hit a place where I cannot find a way to keep jumping around (which would bring me almost right above the crate where the 12th piece is in the cliff). If someone finds a way to go to the 12th piece's "cliff wall" via the right, maybe they can try some stuff to get in there?