I should come out and say that I LOVE Hume, and pretty much think he's on the right lines meta-ethically, even if some of the technicalities of his system now seem a bit crazy (to be fair, he was writing in the 18th century -- we know a lot more about human nature now). I have to admit that I don't quite understand your misgivings about him: for Hume ethics was ALL about 'how we get there'. There are no transcendent 'oughts'. A scientific approach to moral philosophy seeks to understand the emotional processes by which 'oughts' are generated in different societies.
What you say about societies w/o the capacity for empathy is true, but this is hypothetical, right? Human beings have evolved to live in groups, have certain instincts, empathy, because all of these things help us survive. And ethics is a human phenomenon: we can only empirically study what's in front of us. That's about as 'logical' as ethics can get, no?
Basing your entire value system on ethos and the actions of others is even more problematic and arbitrary
Hume argues that we do this all the time, that it's a natural disposition humans have. I think a lot of that makes sense. It's arbitrary from a transcendental point of view, maybe. But if you ground ethics on the real-world experience of how humans behave, then its quite a logical point to make.
Just generally, my sense is that what is so difficult to accept about this stance is that it inevitably leads to some kind of relativism. Criticizing alternative belief structures becomes difficult when you don't have deontological laws or utilitarian calculuses. Then again, the understanding that your ethical beliefs are grounded in illogical, emotional assumptions might also teach humility, and perhaps tolerance as well. You might say (and some have) that this awareness can have a moralizing effect!