Of course you'd need to find a spare soul in order to test that, and Kiyoko(and maybe other companions, I've only read the scene with her in the party) helps establish that Lethicite is not a soul but just a big ole chunk of condensed essence left over from the soul imploding on itself.As there is no guarantee that a demon will cease to be an egotistical bastard whith a soul bound back in them... A drastic and permanent solution might be what has needs to be done.
I would wish to be able to reverse their condition but... stuffing a soul in them is no guarantee they'll change personalities back to what they were before.
I wonder what the ratio is like though, would a Kitsune get a similar meal from eating lethicite as they would from succing someone to death? or is there a net gain from the lethicite at some point in the... refinement.

