Ok, i skimmed that a bit because it was fairly long, but here's a few observations...

I think the default human behavior is to treat what we perceive as simply being what is out there (some people end up learning better, but most seem not to). This is true for everything we percieve, regardless of the subject matter - i.e. is nothing specific to morality.

I think it can -- sometimes -- be reasonable to stand by your intuition even if you can't reason it out. Sometimes it takes time to figure out and articulate the reasoning. I am not trying to justify obstinance and "blind faith" here! Just saying that sometimes you can't be expected to understand it straight away.

I don't see any justification given, in what you quote from Greene, for the claim that there's essentially no justification for morality.