Do all Actors/Actress have Dissociative Identity Disorder ?

I don't think so.  They know the difference between acting & being normal.  They know it is a roll to play & don't mix up the two.  When you hear them talk about a roll they are playing, they always refer to it as the character they portray.

No but all or most actors have above average to high levels of narcissism 

Their personality should change.

Remember they are actors.

I seriously doubt they all have it. That sounds very unlikely.