I personally think that Formless is not the spy, and maybe Brandon is trying to make us think it (I don't know), but I find it very unlikely that Shallan herself would be the spy and she would have absolutely no idea that she killed Ialai. Also, the ghostbloods didn't know about Veil at first, so I'm guessing they didn't know about different personas before hand. Maybe they did. Other then that I don't really know who the spy is, though I do have my suspicions.