Have you seen the XKCD What If on this topic? [1] It doesn't get into the details of éntendue, and even makes a joke about the topic's complexity, so it probably won't satisfy your technical demands. But I've found the arguments about reversibility (light paths through lenses are reversible, so how could one point map back to multiple points?) and area increase (lenses do not intensify image values but can change the overall image size) intuitive enough to help quell myself and my friend's arguments about the matter.