What's the AP cost for cleaning a soul?
Cleaning a soul is the same thing as preparing or processing it. Those are just different terms with the same meaning.
? AP: Remove the abilities of the Drow Fire Elementalist, and graft them onto our own soul (if we can't do that, just make a motion core+abilities Wight out of it (the Fire Elementalist's soul)), if it's reasonably safe to do so.
? AP: Remove the abilities of one of the Dwarven Warriors, and graft them onto the soul of our Tentacle Demon buddy.
5 AP: Make our Tentacle Demon happy by taking it for walksies (explore).
Remaining AP: Train our Elementalism (Can we get a boost by examining what the Dwarf Elementalist soul knows (without harming it, of course)?).
You can't transfer knowledge from a soul to a living being, only between souls. You can teach stuff that you already know to mortal who opens your soul to you.
The souls of Demons (and divine beings) are abnormally structured, and so they cannot be altered the same way you can modify mortal souls. In fact, you don't even know if it is possible.
And, as a side note, your tentacle demon would not benefit from the abilities from a humanoid creature. Different body structures would make it impossible for it to learn from something so different. You can prepare souls from a humanoid body to fit any other humanoid body, but, beyond that, you know that you will have to prepare a soul to be a Chimeric Abomination to fit an animalistic or oddly shaped body.