His argument is based on the fact that since Bnei Noah are commanded in Dinim, their Din is valid even for two Jews. The point I would like to make is that even the Or Zarua speaks only about the validity of such going to Arkhaoth. But he also admits that in having chosen these Arkhaoth the parties have transgressed the issur of . Were the Or Zarua speaking only of a corrupt system, it is hard to say that he would grant such a system any validity. Rambam, Hilchos Sanhedrin, 26,7.
Some versions have it as , instead of .
The Halacha applies to both. See note 6. Shulchan Aruch, Choshen Mishpat siman 60, s'eif 1. For discussion of the parameters of Dina Demalchusa Dina see article by Rabbi Herschel Schacter - Journal of Halacha and Contemporary Society, Vol. 1, no. 1. Choshen Mishpat siman 26.