In the example of Nazi 'morality', I am claiming that there is a logical incoherence in the idea that there could be an empirical basis for circumscribing the circle of 'persons who count'. You seem to have missed the crux of the argument here. What the other's interests are is a factual question. This is something the successful amoralist would need to know in order to be successful in manipulating the other to the amoralist's own ends. Considered purely as attitudes, moreover, caring or not caring about another's interests is equally 'rational'.
Solipsism is incoherent.
The rejection of solipsism entails recognition of the 'transcendent world of the other'. Recognition of the transcendent world of the other cannot be equated with recognising the truth of a statement. It is not a piece of theoretical knowledge. The only alternative possibility for making sense of such recognition is in my actions towards the other. It is essentially a matter of practical knowledge.