I agree completely on the difference between pain and suffering, and that being alive means experiencing pain; but if we're talking about pain as grounds for establishing ethical behavior, I'm not sure I see your point.
I feel pain while running, for example, but that's pain I'm willingly accepting due to its benefits. I can't think of a case in which someone inflicting physical pain on my body is ethically acceptable unless we've established so beforehand. I don't think "spare the rod, spoil the child" is an ethically sound position, and firmly believe it's a sign of weakness and impatience on a parent/guardian's part.
So the point isn't that pain is inherently unethical, if we want to return to your language about people not inherently deserving respect. As I said, I don't think it's the case that people do inherently deserve respect. But if we're talking about the ethics of interpersonal relationships, then the knowledge that one is causing pain is, I think, a fine place to begin reflecting on whether one is acting ethically.
So to pose it in simpler language, I don't see at all why pain is a "terrible point of focus" when discussing ethical responsibility. Nothing about ethics is inherent, absolute, universal, etc. But we have to start somewhere, and I don't understand why pain isn't a good place to do so.