So, a couple of things here. First off, I'll take any knocks I get--and try to do better. I'm not an expert on mental health, and though I do my best, I'm going to get things wrong. I'm going to risk defending myself here--and hopefully not dig myself deeper--as I at least explain my thought process, and why I built Shallan the way I did.
However, one of the rules of thumb I go by is this: individual experience can defy the standard, if I understand that is what I'm doing. Like how Stephen Leeds is not trying to accurately portray schizophrenia, Shallan is not trying to accurately portray dissociative identity disorder (if a scholarly consensus on such a thing even exists. I haven't glanced through the DSM5 to see what it says.)
In Legion, I have an easy out. I say, point blank, "He doesn't fit the diagnosis--he's not a schizophrenic, or if he is, he's a very weird one." I don't have the benefit of a modern psychology voice in the Stormlight books to hang a lantern on this, but my intention is the same. What Shallan has is related to her individual interaction with the world, her past, and the magic.
Is this Hollywood MPD? I'm not convinced. Hollywood MPD (with DSM4 backing it up, I believe) tends to involve things like a person feeling like they're possessed, and completely out of control. The different identities don't remember what others did. It's a very werewolf type thing. You wake up, and learn that another version of you took over your body and went out and committed crimes or whatever.
Shallan is coping with her pain in (best I've been able to do) a very realistic way, by boxing off and retreating and putting on a mask of humor and false "everything is okay" attitudes. But she has magical abilities that nobody in this world has, including the ability to put on masks that change the way everyone perceives her. She's playing roles as she puts them on, but I make it very clear (with deliberate slip-ups of self-reference in the prose) that it's always Shallan in there, and she's specifically playing this role because it lets her ignore the things she doesn't want to face.
She's losing control of what is real and what isn't--partially because she can't decide who she wants to be, who she should be, and what the world wants her to be. But it's not like other personalities are creeping in from a fractured psyche. She's hiding behind masks, and creates each role for herself to act in an attempt to solve a perceived shortcoming in herself. She literally sketched out Veil and thought, "Yup, I'm going to become that person now." Because Veil would have never been tricked into caring about her father; she would have been too wise for that.
I feel it's as close as I can get to realism, while the same time acknowledging that as a fantasy author, one of my primary goals is to explore the human interaction with the supernatural. The "What ifs" of magic. What if a person who had suffered a great deal of abuse as a child COULD create a mask for themselves, changing themselves into someone stronger (or more street-smart who wouldn't have been betrayed that way. Would they do it, and hide behind that mask? What would that do to them and the world around them?
DID is indeed controversial, but I really like this portrayal. Not of a disease, but of who this character is. And I've had had enough positive responses from people who feel their own psychology is similar that I'm confident a non-insignificant number of people out there identify with what she's doing in the same way people with depression identify with Kaladin.