"Emily and the Gray Man were in fact two different individuals." - David Whiting / by H.P. Mendoza

My main conclusion was that Emily and the Gray Man were in fact two different individuals and though Emily may have stabbed herself to death in an episode of her illness, the Gray Man was about to attack her and that may have triggered the episode. That's why we see the Gray Man with his own eyes as opposed to black voids. Unclear as to the identity of the Gray Man, however I felt he was not a part of Emily's illness created blackouts. I also feel Sylvia was in over her head. At the end, she abandoned Emily in her own fright over the revelation of the Gray Man. In the final scene, it's Emily who must confront everything. Her illness, her death, the Gray Man, the encroaching darkness (not "The Light", as promised).