How should I think about whether a troubling behavior is a "disability trait" or "willfulness"?

"Willfulness" and "traits" look alike, and telling them apart isn't easy. A clue is observing when and in what situations it happens. If specific sounds, changes, fatigue, or being unable to communicate are triggers, it's more likely an expression of a trait or a difficulty. What matters is looking at the background before deciding on a cause. Adjusting the environment and how you communicate to fit the background settles many behaviors. Rather than forcing a black-and-white "which is it," the stance of thinking together about "what is the child struggling with" helps.