Question: Why don't we ask from the Beraisa which says "which is the second Shabbos? It is the one in which Purim falls, even on Erev Shabbos."

According to Rav, it could (i.e. should) have taught "[the Shabbos before Purim], even [if Purim is] the next Shabbos", and [we would know that] all the more so if Purim is Erev Shabbos, before the next Shabbos, we advance [Parshas Zachor] to the previous Shabbos!

Answer: This Beraisa is the Beraisa below, after the Mishnah "on the fourth we read "this month";

Since it taught "even on Erev Shabbos" regarding the first and fourth Shabbos, which is precise, it taught also here (the second Shabbos) on Erev Shabbos, and not on Shabbos, even though it is not precise. (Really, the second Shabbos is the Shabbos before Purim, even if Purim is the next Shabbos.)

Implied question: [On Yom Tov and Chol ha'Mo'ed Pesach,] we take a second Sefer Torah, and read the Korbanos of the day. There is no source in the Gemara for this!

Answer: This is in Seder Rav Amram Gaon.

Support: It says below (31b) that Avraham asked Hash-m... Hash-m answered, I enacted for them the Seder of Korbanos. Whenever they read them...