However I have seen little discussion of the earlier part of the spectrum (friends and family vs. strangers), and it seems to be the one on which our intuitions agree the most reliably - which is why I think it deserves more of our attention (and having clear ideas about it might help about the rest).

I think, like you point out, this gets into near / far issues. How I behave around my family is tied into a lot of near mode things, and how I direct my charitable dollars is tied into a lot of far mode things. It's easy to talk far mode in an abstract way (Is it better to donate to ease German suffering or Somali suffering?) than it is to talk near mode in an abstract way (What is the optimal period for calling your mother?).

This was a big debate in ancient China, between the Confucians who considered it normal to have “care with distinctions” (愛有差等), whereas Mozi preached “universal love” (兼愛) in opposition to that, claiming that care with distinctions was a source of conflict and injustice.

The Spring and Autumn period definitely seems relevant, and I think someone could get a lot of interesting posts out of it.