I don't think Sarl is much more than he appears...a crazed Scalper. I think that at the end of WLW he's possessed by the Captain's soul, which has 'bounced'. I think his killing the skin spy just when it might have spoken up is just bad fortune.
The skin spy does get orders from the Synthese, right? I don't think it had a human handler beyond those orders. Just there to protect Mimara and make sure she fulfills whatever prophecy, false or real, they want to bring about.
Worth considering that the Consult may be fans of Akka right now, if they know of him.