Question- Why does Rav hold that 2 can make a Mezumen if they wish? The source for Mezuzen from Gadlu or Ki Shem specifically deals with 3 people. What about Motzei Shem LiVatala?
Chaim Ozer
1) The verse is only referring to when making a Zimun is obligatory -- with three. Rav only says that two may voluntarily make a Zimun, but they are not obligated to. Besides this, it seems that the verse is not a d'Oraisa source for Zimun, but only an Asmachta. Hence, Rav has greater liberty to differ.
2) The prohibition of saying Hash-m's name in vain is only when it is just that -- in vain. But here, they are praising Hash-m, so therefore it is not in vain. Only a blessing in vain is forbidden (even though it is for the sake of praising Hash-m), but here, this is not a blessing.
Kollel Iyun Hadaf