John, that is an interesting example. While I agree with your reasoning, let me introduce a twist.
When I was in Yosemite, I had a full moon in the scene. Given the inherent dynamic range limitations of slide film, I got the scene mostly right, but the moon overexposed. I had another camera with me, loaded with a negative film, and I snapped the same scene perhaps minutes apart. Now, I was thinking of taking the moon from the negative film (which has a greater dynamic range) and photoshopping it into the slide film scene. This would satisfy your requirement ("Does this represent what actually happened out there?"), yet it will be not only photoshopping, but compositing as well. So, if I say "Yes," would I be unethical?
I'd have no problem with it at all...if you explained it as you just did. Because you're disclosing the manipulation. One thing about these borderline cases -- if you showed me a photo taken at night, that had a lot of detail, and also a well-exposed moon (the moon being really bright) I'd say hmmmm...something ain't right. You can't do both of those things, not even with the widest latitude film. You couldn't dodge the moon because it would have been blown. So I'd know you manipulated it somehow, and I'd wonder what else had been done. Is the moon really coming up like that? Or did you take a shot of a mountain off to the north, and have the moon coming up in the north? If you explained it by saying, "I knew I couldn't hold this scene as I saw it, so I took two shots, from the same vantage point, a few seconds apart, and substituted the well-exposed moon for the blown one." I'd probably say, "That sounds legitimate -- you're not changing the scene, you're attempting to defeat the shortcomings of the sensor." If I liked it, I'd buy it. But with all that, you've done two things -- you've made a scenic change so minor that not even an astrophysicist could detect it, and you've disclosed the manipulation. However, if you'd taken that shot in Yosemite, and then a really good moon shot out over the ocean from Santa Barbara, and composited the moon, I wouldn't buy it, even if you disclosed. By disclosing, you wouldn't have done anything unethical with the Santa Monica moon, but when I collect art photos (and I do) I want actuality, as close as I can get it. I'd argue that your Yosemite photo was actuality, as close as you could get it, and the disclosure of manipulation eliminates any ethical problem.
Look, I have a very nice Moonrise Hernandez NM print, and Ansel manipulated the hell out of it. I have no problem, because he disclosed, and he did not change the physical elements in the photo. Some of his prints, in which the sky is brighter, might be the way you'd see it with your eyes wide open; the later ones, with the darker sky, is like you were squinting. But he didn't add any cemetery crosses, or put in a shooting star...
Again, if we're talking about ethics, the ethics lie in the intent of the photographer when presenting the work. If the intent is deliberate deception, with perhaps a monetary motive, then it's unethical.