Going back further...James Dean and Marilyn Monroe. I doubt either would be talked about very much today if not for the "tragic death at a young age" angle. Bruce Lee also benefited from this.
And you're right about MJ. Last year he didn't even make the top 100 on the Forbes earnings list...this year he jumped up to the top 5. Wonder how that happened?
Yes, sadly the death of stars is an huge marketing (I'm a really big fan of Bruce Lee too "JeetKuneDo"

), these companies take lot of advantage about this situation, and logically, Warner have taken the opportunity to promote the Dark Knight with Ledger death, and it was a really big part of the success.
For the Dark Knight, there are also the fact that we don't have seen the Joker since the first Batman movie who have a huge reputation among the fans, with his death, it was another important point for the huge success of the movie, Joker is iconic, probably one of the most well know comics character, the tragic death of a young actor in the iconic last role, it's simply an important, if not the most important commercial aspect for this movie.
For Iron Man box office, I think his success is more important that the others superheroes movies, simply because, unlike the others popular franchise superheroes, nobody knew him before his movie.
Batman, the X-men, Spider-man, Superman, Hulk, each of this characters was really well-known in the pop culture (with TV show, cartoon etc...), but Iron Man, nobody know him, in France before this movie, nobody, except some comicbook fans, knew who he was or who was Tony Stark, in other hand, everybody know Batman and Spider-man.
So I think that Iron Man was the most successful because he was near nobody in term of popularity, but now he is maybe, after Spider-man, the most well-known Marvel character.