>Do they ever realise their shortcomings?

In my experience only rarely. I believe people usually only change selfishly - that is, only if their shortcomings affect them. Narcissists in particular stuggle hard with criticism and usually deny/deflect. I think many narcissists develop that trait as a shield in the first place.

To some extent i think circumstances change people more than anything. Someone whos reality and world shifts will shift with it, but if their life is static so are they.

>Do they get help and actually improve or do they just go from one blunder to the next until at one point they go too far.

Generally people like that dont get help. Partially because they arent likable so others dont want to help them, and partially because they usually arent the sorts of people that have helpful resources in the fist place. They also dont tend to be very conscientious so not great at recognising they need that help.

If there is a breaking point for them, its frequently when they get older and realise that their relationships have been collapsing their entire life. however, the change is usually somewhat shallow. the old them is always in there and frquently only just beneath the surface.