(Please forgive the delay in response. Technical problems prevented the mailing of a number of responses.)

1) It seems that one must say that the Gemara Shabbos is referring to a question that was asked after the time of Rav and Shmuel. When the Gemara writes "Iba'aya Lehu," Rashi sometimes explains that this was a question asked by the "Bnei ha'Yeshiva," the students in the Yeshivah. Sefer ha'Kerisus (by Rabeinu Shimshon of Kinon, one of the Ba'alei Tosfos) writes (Lashon Limudim, Sha'ar 3, Os 76) that every time the Gemara says "Iba'aya Lehu" it refers either to the Bnei ha'Yeshiva or to the Chachamim who specialised in this Halachah. In addition, the Shelah ha'Kodesh writes (Shnei Luchos ha'Bris, Torah she'Ba'al Peh, Klal Yeshiva, #161) that every Stam "Iba'aya Lehu" in the Talmud means that the Bnei ha'Yeshiva asked this question. This suggests that they were Talmidim of the Yeshiva close to the time of the completion of the Gemara by Rav Ashi. In contrast, Rav and Shmuel -- who had the dispute in Avodah Zarah 2a -- lived at the very beginning of the time of the Gemara, shortly after the time of the Tana'im of the Mishnah. Therefore, the Gemara in Shabbos (77a) did not ask about "Edeihen," because this was a well-known question from several generations earlier, and since Rav and Shmuel were not able to decide what the correct text was, the Bnei ha'Yeshiva at a much later period did not expect to receive an answer to this difficult and well-known question.

2) In addition, the Gemara in Avodah Zarah (2a) states that neither Rav nor Shmuel made a mistake, but in fact the text of both of them was a plausible one. Therefore, if the Gemara Shabbos would have asked if the text of "Edeihen" was with an Alef or an Ayin, the Gemara would have had to answer that both are acceptable possibilities. This is not similar to the other examples that the Gemara mentioned in Shabbos, where there is one clear answer in each case.

Kol Tuv,

Dovid Bloom