I'm not sure what our disagreement actually is - I agree with your summary of Ayn Rand, I agree that there are lots of ways to hurt people without stabbing.  I'm not sure you're claiming this, but I think that failure to help is selfish too, though I'm not sure it's comparable with active harm.  

It may be that I'm reacting badly to the use of "truly selfish" - I fear a motte-and-bailey argument is coming, where we define it loosely, and then categorize actions inconsistently as "truly selfish" only in extremes, but then try to define policy to cover far more things.  

I think we're agreed that the world contains a range of motivated behaviors, from sadistic psychopaths (who have NEGATIVE nonzero terms for others' happiness) to saints (whose utility functions weight very heavily toward other's happiness over their own).  I don't know if we agree that "second-order effect" very often dominate the observed behaviors over most of this range.  I hope we agree that almost everyone changes their behavior to some extent based on visible incentives.

I still disagree with your post that a coefficient of 0 for you in someone's mind implies murder for pocket change.  And I disagree with the implication that murder for pocket change is impossible even if the coefficient is above 0 - circumstances matter more than innate utility function.

To the OP's point, it's hard to know how to accomplish "make people less selfish", but "make the environment more conducive to positive-sum choices so selfish people take cooperative actions" is quite feasible.