Mraize - Shallan
Not sure if that theory has already been discussed. What if Mraize lightweaver's spy is actually Shallan herself, or more another of her persona. Maybe the formeless one ? That would mean she actually killed Ialai . To push further, the ghostbloods may know about that persona since Shallan childhood and that would explain why Mraize seems to know so much about her past.