Answer: A.      In the Shulchan Oruch of Admor HaZakein 189:64, it says: “When a non-Jew is present in the house, we are accustomed to say “kulanu yachad bnei bris” (“all of us together are members of the covenant”) in order to exclude the non-Jew from the blessing, for it is forbidden to bless the non-Jew, since it is written “you shall not grant them grace.” It is more correct to say “bnei bris kulanu yachad” so that there will not be a given moment when we say “kulanu yachad”, (all of us together”) before finishing “bnei bris,” (“members of the covenant.”) B.      In the responsa of the Minchas Yitzchak (in Likutei Tshuvos 129:9) he proves that it should be said specifically when the non-Jew is in the room where the Birkas HaMazon is being said, and not if he is just present in another room. His opinion is presented in the Likutei Tshuvos, Letter 3:14.