(a) Question: Rabbah (who maintains the Torah prohibition of Hachanah) seems to be the only understanding of our Mishnah which also explains why a Safek egg is Asur and will not be Batel, among the Halachos cited in the Beraisa (whereas Mashkin she'Zavu and Peiros ha'Noshrin are Gezeiros d'Rabanan, and should be Mutar b'Safek)!?
WHY? I don't understand, even though Mashkin she'Zavu and Peiros ha'Noshrin are Gezeiros d'Rabanan however, it is a Davar she'Yesh lo Matirin and is therefore Asur! So what is the Gamorah's question: it only makes sense according to Rabbah who maintains it is a Torah prohibition. A Safek egg should be Asur across the board because it is a Davar she'Yesh lo Matirin.
RE, spring valley, ny
Dear RE,
Davar she'Yesh Lo Matirim does not become Batel in a mixture - as the Gemara says, "Afilu b'Elef Lo Batel." However the Gemara is not discussing a mixture at this point. It is discussing a single item about which we have a Safek. In this case, we assume that the Halachah should be that it is Mutar since a Safek d'Rabanan is normally l'Kula.
Only Rav Ashi (Daf 4a) is Machmir by Davar she'Yesh Lo Matirim with regard to Safek d'Rabanan just as Machmir by Davar she'Yesh Lo Matirim with regard to Bitul - like you assumed. (See Rashi 4a DH Afilu mid'Rabanan.)
All the best,
Reuven Weiner