Haven't read the novel either but that's my theory too. The weird look that Honey gives him when he calls himself a human makes me think that either the human Seo Jio merged with the entity in the painting, or he changed after staying the painting for so long?
But he himself doesn't seem to have realised it
I don't think it's an entity, but the latter might also be possible. I think it's also a manifestation of his love for painting and I think the root also played a part in it.
Root definitely plays at part in it I believe
The mourning, the white flowers for a funeral, and the feeling of the "self" being stripped away. Does that mean that Jio has been turned into a being? Which explains why he looks like a monster or even a god or the root's children to others. He's still human or at least he still remembers and views himself as a human but not quite.