Our evolved notions of disgust are there for complex reasons and they cannot be analyzed exclusively as helping society to solve group co-ordination.

It doesn't need to be analyzed exclusively that way; it's just one reason it's there, a sort of focal point. "Eww! That guy eats snakes, he's not like us [which will make it harder to punish him for defection]" Even if the reason for a tradition changes, it can still serve as a focal point to identify ingroup/outgroup.

point about family breakdown seems like a classic example of post-hoc justification if ever I saw one.

It wasn't my point, I was just parroting someone else I read on the matter.

In any case, I wasn't endorsing the response I gave to the incest case. I was just showing what a response would look like from someone who was actually prepared. When people aren't prepared for a question -- i.e. the 99% of the population that doesn't deeply reflect on their aversion to incest -- yes, they'll defend a position despite lack of a justification. But guess what: you'll get the same thing if you ask people to justify their belief that the earth is round.

Greene reads too much into this failure to offer a justification for unusual dilemmas.

Consider also the trolley cases. Why did evolution equip us with a tendency to avoid pushing the man off the bridge to save 5? What on earth does that have to do with prisoner's dilemma?

Again, I ask that you look at this from the perspective of an actual participant in the survey. That person is imagining grabbing a random person and tossing them off a bridge on very short notice. Numerous factors come into play, and the participant is going to consider them whether or not you assure them that they don't matter.

Anyway, there are separate questions here:

1) Why did evolution equip us not to push someone off a bridge ...? Because overt murder is a bad strategy, and the benefit isn't tangible enough to outweigh it.

2) Why do people, on sober reflection of the issues, still consider it unethical to push the fat guy off the bridge? That's easy. For one thing, people on a trolly consented to the risk of a crash in a way that someone standing on a bridge did not consent to some psycho f--- suddenly deciding to push him off out of some bizarre sense of heroism. They also intuitively see such an extreme action as violating norms, which makes future actions harder to plan ("let's take the long way around the bridge"). Etc. There are many reasons to distinguish the alternatives that the "clever" people who design the surveys aren't taking into account.