Volvie-
We aren’t relying on the non-Jew’s nemanus. We are relying on what is reasonable to assume. There is no need for eidus, so nemanus is irrelevant. This is commonplace in halacha.
That which you and others claim that R’ Moshe’s psak was only b’shaas hadchak is something both of his own sons, R’ Dovid and R’ Reuven disagree with. They both hold, besheim their father, that c.s. is lechatchila.