Just dropping some thoughts regarding the material available for the ending.
Whilst none of it particularly conflicts with the outcome that Willard completes his mission.
I think that at one time, the intention was although you see Willard complete his mission in every version the intention may have been that the viewer was not meant to be sure if it was real or imagined. There is a sense of that in the film but it is not as pronounced as it perhaps could have been or should have been.
This I have concluded because of the flash forwards at the start of the film. The prostitute by the window blinds although we don’t see willard in bed with her has the same posture and squatting as a woman straight after kurtz is killed mourning.
So within the compund should have been more surreal than it already is.
But although the callbacks to the start montage are there there at “the end” there is no outro montage as such a bit is there though. I think this was really the intention at one time to be more of a surreal montage so you were not sure if it was real or not within the moment the past or the future But it never got to the same level as the opening section montage. But the seeds of it are there within the story. Showing images Kurtz alive in the shadows after he was killed would achieve this in full. Showing willard escape but then still be trapped in a cage. Colby eating bugs after his death and so on.
Anyway this may have been the creative difference and was chosen to play it down and more straight and conclusive… Rather than Inconclusive and delibetately confused but with clarity enough within that confusion willard completed his mission however real or imagined it was is up to the individual to decide.
Edit:
Something also springs to mind when think8ng of the opening montage. Going off or riffing off hearts of darkness and Willard feeling that the home he knew was not there anymore and the imposing one culture on another instant gratification theme. Something that would have really rammed the theme home would have been a city skyline dissolving into the napalm strike footage. Willard was divorced that day from his wife hence the prostitute I guess.
Bookending this at the end Willard took Kurtz notes and writings and was to originally either speak with Kutz Wife or Son (ending had about 10 rewrites). Willard was to lie and not be entirely truthful about what happened to Kurtz or his last words to family. I don’t think this ending would have worked at all yet Willard still takes the writings.
This again is paradoxical as he has left the army no need to prove anything to his superiors yet takes the notes still he is also not going to Speak to Kurtz son or wife for that matter. So the notes are taken to be… Destroyed?
Kurtz wanted death but his writings to potentially live on past his moment of realization as he died.
This is why I think it should have played the slug on the razor blade speech angle with Willard dreamed it up in a cage after escape, Kurtz still lives after death the camp is bombed as the tribe dances. All opaque. Like it all happened and it all never happened. The truth is that nobody will ever know anyway and that is the point nobody is supposed to know. The mission never existed Nothing would be resolved which ever way you look at it. There was no redemption. Kurtz does not exist in the land of city skyscrapers “real world” neither does Willard.
But whatever did happen would just be a conspiracy theory there would be no reality that accepted the truth. So it must all be a lie. So the truth is that it is all lies. Kurtz says as much and his death should be treated no diffetent as should Willards escape and living on…all lies.