Brachos 50 - Intending not to make a zimun

Rav Moshe (Igros Moshe 1:56) has a teshuva about people who are attending a wedding and don't want to stay around until the end for the sheva brachos. The question is that all the attendees have both an obligation of sheva brachos and an obligation of zimun, so how is anyone allowed to leave a wedding early? Rav Moshe explains that even if people sit together so that there is a kevius in their eating, if they are not intending to create a kevius for the bracha because they don't plan to bentch together, they are not obligated in zimun. His source is from the Rama (193:3) who says that when a group eats together in the house of a goy, the assumption is that they intend not to join for birchas hamazon (because skipping the harachaman for the ba'al habayis will be noticed and cause animosity). The Magen Avrohom in Hilchos Tisha B'av (552:9) uses this Rama to explain why one doesn't make a zimun for the seudah hamafsekes even when 3 people ate together. Since their intention when eating together is not to join for the bracha, they are not obligated in zimun. Rav Moshe explains that when there is no kevius for the bracha to obligate zimun, there is also no obligation to stay for sheva brachos.

At the end of the teshuva, Rav Moshe explains that with this yesod we can offer a better peshat in our gemara. Rava says that when they ate at the home of the Reish Gelusa they would make a zimun of groups of 3 quietly, and not wait for the Reish Gelusa. The gemara explains that due to the large crowd they wouldn't be able to hear the bentching from the reish gelusa, and to form groups of 10 would be obvious and anger the Reish Gelusa, so they formed groups of 3. The question is, how were they entitled to forfeit the obligation to make zimun with 10 people, just to avoid angering the Reish Gelusa. The Magen Avrohom writes that it would be disrespectful to the Reish Gelusa not to wait for him, so we allow kavod habriyos to override the obligation to bentch with a zimun of 10. However, in light of Rav Moshe's chiddush that when one never intends to be part of the zimun there is no obligation, he explains that we don't need to say that the kavod habriyos overrides the obligation of zimun. Rather, since initially the fully planned on breaking into smaller groups of 3 and not joining to be part of groups of 10, there never was an obligation of zimun on them to be mezamein with 10. Rav Moshe points out that since there is another explanation as to why they were able to avoid the zimun of 10 (kavod habriyos), there is no proof to the opinion of the Rama that intent to not join enables one to avoid the zimun. But in light of the Rama, we have a better explanation for the gemara.