I agree with this what you say wholeheartedly. I feel they're showing the complexity of dealing with trauma, and Shauna is showing a good bit of the angry, paranoid, and dissociative side. With viewing adult Shauna and teen Shauna side by side, I wouldn't be shocked if she has some form of EUPD/BPD from everything she went through. They all obviously have C-PTSD paired with dissociative tendencies, but Shauna's character, I feel really is showing the depth on all the realities that come with trying to hide what you went through and faking normal pretty well.