(1) In the question to Insights 7b you mention that the gemorah concludes that one needs and "eruv chatzeros" - shouldn't it be a "shituf mavo'os"?
(2) In the Answer - Your conclusion about Rashi's intention is excellent. Actually, if one is exact in Rashi's words, one can see this clearly. Rashi states that a mavoy doesn't forbid "chatzeros" open to it, but "chatzeros" forbid a movoy. Clearly this is talking about cases in general, as you said, for in our case there are not "chatzeros" only one chatzer.
(3) Question - In the movoy akum of Sura - is the conclusion that it the budia is nailed in that one can carry in the mavoy only according to Shmuel, or would Rav permit it also?
Reuvan
(1) Yes, it the people of the Mavoy make a Shituf with those of the Rechavah. The Gemara though, does call it an Eruv ("she'Lo Irvu"). (Normally, a Shituf is made between two Chatzeros that lead into a Mavoy, and not between people living in the Mavoy itself and people in a Chatzer that leads to it.)
(2) Yasher Kochach
(3) The simple understanding is that the Budya, or mat, that was placed in the bend of the Mavoy Akum in Sura permitted the Mavoy only according to Shmuel. Rav would require a Tzuras ha'Pesach there. However, if the two other sides of that Mavoy each had a Tzuras ha'Pesach (and not just Lechi), then Rav would agree that a Lechi (such as a nailed-in upright mat) in the bend would be sufficient. Alternatively, if the members of one half of that Mavoy did not want to make a Tikun on their side opening into Reshus ha'Rabim, it suffices for the members of the other half of the Mavoy Akum (which is considered its own Mavoy) to put up a Tzuras ha'Pesach on one side and a Lechi on the other, like any Mavoy Mefulash. So that case in Sura could be according to Rav as well. (See also ROSH to Daf 6a.)
Y. SHAW