^ Yeah, I think ambiguity was the intent. The version of the movie I "grew up with" ended with the Kurtz compound destroyed in many slow-motion explosions. I always assumed that was Willard's airstrike, but apparently that was not Coppola's intention; he meant it as something more abstract and cut that ending from further releases.
I think Lance had lost his mind, I don't think he had much of a future.
For Willard, I think his faith in everything had been shattered; whether he stayed in the army, went home or whatever, he had a long way to go to feel like he "fit" in the world. I think the ambiguous ending for him is good in that way.
FWIW, Marlow went home and spared Kurtz's fiancee the gory details, but he also gave a reporter the full story.