Sadly the couple were not well suited so their marriage was not likely to survive even if neither had avoided their respective affairs. While I do agree that Diana was too young and neurotic to be involved in such a public and high profile marriage,
I'm not sure that "damaged" is the best way to describe Diana. However I must say that her parents' fractious divorce definitely took its toll upon the emotional well being of Sarah, Diana and Charles. Jane seemed to be the only child who was relatively unscathed by it.
I know it is a bit of a blunt wrod, but I think it does apply to Diana and to an extent to other Spencer children. as you say Charles and Sarah S were also wounded by the parents divorce and after events. And I think that although Diana did have therapy during her marriage, she seemed to end up if anything more paranoid and mixed up as she grew older, as witnessed by the events preceding the Panorama interview...