You wrote in the Point by Point Summary:
>>(a) (Beraisa #3): Anyone may recite Megilas Esther (and discharge the obligation for adults who hear him), except for a Cheresh, Shoteh (lunatic), or Katan (minor);
(b) R. Yehudah permits a Katan.
(c) Question: Who is the first Tana, who invalidates a Cheresh even b'Di'eved?
(d) Answer #1 (Rav Masnah): It is R. Yosi:
1. (Mishnah): If one said Keri'as Shema inaudibly (to his own ears), he was Yotzei;
2. R. Yosi says, he was not Yotzei.<<
My question is, I understood that R. Yosi makes a diffenrece between deoraita and rabbanan. In rabbanan bediavad its ok. So why do we say here by Megila (which is Rabbanan) that its R. Yosi?
1. Notice that the Gemara establishes the author of the Mishnah in Megilah as Rebbi Yosi only after having established the Mishnah in Terumos as Rebbi Yehudah. That being the case, Rebbi Yosi will no longer differentiate between d'Oraisa and d'Rabanan.
2. Your question will then pertain to the Havah Amina. In view of the Mishnah in Megilah, how could Rav Chisda even think that Rebbi Yosi differentiates...? The answer is that, granted, the Gemara could have asked that question. But since it anyway has arrived at the conclusion that the Mishnah in Terumos is Rebbi Yehudah and not Rebbi Yosi, this is no longer necessary.
3. The above is according to the Sugya in Berachos. The Gemara in Megilah, however, concludes that the author of the Mishnah there is Rebbi Yehudah, and the reason why a Katan cannot be Motzi even b'Di'eved is because it is talking about a Katan who has not yet reached the age of Chinuch. In that case, Rav Chisda may be correct when he says that Rebbi Yosi's ruling is confined to a d'Oraisa, but that mid'Rabanan he agrees that b'Di'eved one is Yotzei.
Wishing you and yours a G'mar Chasimah Tovah,