A well-written and clear original post. I have two major criticisms 1) You’ve failed to make your case and 2) Your claim is wrong about human nature.
I think the weakest part of your argument is that it’s circular. You define everyday actions as being motivated for personal benefit and then use that as evidence for your claim that the primary human motivation is for personal benefit. The one that struck me most:
So, sincere and heart-felt care and interest for others is evidence for action in self-interest. If that’s your claim, then no argument in opposition to yours has a chance.
Your description of human social history is a caricature—seems-to-me history. Again, you describe so-called history slanted toward your position and claim it as evidence. Your purported evidence for your position is “a large number of life examples, observations, and analysis of cause-and-effect relationships. Such patterns have a statistical character,” but you haven’t provided any specific testable claim at all beyond your own personal—and uncharitable—interpretation of everyday human behavior.
In my personal experience, concern for the interests of others is a major motivator for human behavior. Of course there are plenty of examples of exploitation of others for personal benefit. As I understand human nature based on reading, observation, and introspection, humans have evolved as social animals. We like and like to be with each other. How could it be any different. No society, no family, could survive without concern for others. Everyday I see people treating others with kindness and concern. We like each other. We like to hang around with each other. We feel a sense of responsibility for others.