She wasn't merely delusional, she was hallucinatory---she was flagrantly psychotic, and had been, intermittently, for years. (ETA: Sorry-not jumping on you; this is a case that has frustrated me since its inception; I know AY's psychiatrist, and the whole horrible ordeal could have--and should have--been completely avoided. Personally, I believe her ex-husband belongs in prison. But I digress...)
While sociopaths can have comorbidities---can lapse into delusion or even psychosis--you've hit the nail on the head. The crux of it is that Casey's active long-term coverup, during which she consistently maintained the same story (showing an organized thought pattern), completely wipes out any chance of even a momentary entertainment of the notion of applying an insanity defense.