Well-being is what matters, but needn't exhaust the moral facts
Tangential: I'm curious whether Scanlon has ever answered the immediately salient question of how he might explain what "could reasonably reject" means? Is the idea that the answer to what that means will appeal to moral ideas like rights, fairness, justice, oppression, desert, etc., and that the apparently blatant circularity is not a problem because he's a "coherentist" and he believes "thick" moral concepts or something?
I don’t think I understand the robots and paper clips case. My reason for helping the robots find the paper clips is that this will secure their help saving drowning kids. The fact the robots care about clips is itself no reason for me to help them. It is the fact that the robots will help me if I help them that is my reason. My reason for saving drowning kids is that they are drowning. The fact that they are drowning is reason to help because (says Scanlon) a person could reasonably reject an argument that gave that fact no weight. Are you assuming the robots are persons?