MUST ONE INTERRUPT?
Answer #1 (Mar Zutra): Perhaps Beraisa #2 is erroneous (it should say 'after nine and a half hours', like the simple understanding of Beraisa #1)!
Rejection (Mereimar): A reciter of Beraisos said it in front of Rav Pinchas brei d'Rav Ami, and he accepted it! (Surely, he had a solid tradition that it is correct - otherwise, we could say that Rav Papa argues with him!)
Question: If so, Beraisa #2 refutes Rav Papa!
Answer: Indeed, [it does -] we must answer like Rav Huna (see note 55 in Appendix).
Question (against Rav Huna): R. Yirmeyah taught that the Halachah follows R. Yehudah regarding Erev Pesach, and the Halachah follows R. Yosi regarding Erev Shabbos;
This implies that they argue about both of them!
Answer: R. Yirmeyah taught about whom the Halachah follows regarding interrupting:
(Beraisa - R. Yehudah): On Erev Shabbos [if one started eating when it was permitted, and it became dark,] he must interrupt (Rashbam - make Kidush; Tosfos - remove the table to say Birkas ha'Mazon);
R. Yosi says, he need not interrupt.
A case occurred, R. Shimon ben Gamliel, R. Yehudah and R. Yosi were eating in Ako; Shabbos came. R. Shimon ben Gamliel suggested to R. Yosi that they should be concerned for R. Yehudah's opinion.
R. Yosi: You always try to get R. Yehudah to agree with my opinion - why now do you try to get me to agree to his opinion (this is shameful to me)!
R. Shimon: If so (you do not agree), we will not interrupt, lest Talmidim see and the Halachah will be established like him.
They did not leave until the Halachah was established like R. Yosi.
(Rav Yehudah citing Shmuel): The Halachah does not follow R. Yehudah or R. Yosi - rather, one spreads a tablecloth and makes Kidush [then continues eating. Rashbam - really, he holds that the Halachah follows R. Yosi - he was extra stringent; Tosfos - he holds like the Beraisa on Amud B; spreading a tablecloth is in place of removing the table.]
Question: But Rav Tachlifa bar Avdimi cited Shmuel to say that just like we interrupt for Kidush, we interrupt for Havdalah.
Suggestion: We interrupt by removing the table (they used to have a small table in front of each person; it was removed before saying Birkas ha'Mazon)!
Answer: No, we interrupt by spreading a cloth.
Rabah bar Rav Huna visited the Reish Galusa's house; they brought a tray in front of him (Rashbam - at the beginning of the meal; Rashi - he was eating, and Shabbos came); he spread a cloth and made Kidush.
Support (Beraisa): We do not bring a table [in front of a person] unless he already made Kidush; if a table was brought, he spreads a cloth and makes Kidush.
WHEN MAY ONE START?
(Beraisa #1): They (R. Yehudah and R. Yosi, who argue about interrupting) agree that one may not start [to eat after the time of Minchah].
(Beraisa #2): They agree that one may start.
Question: We understand Beraisa #1 - they agree that one may not start on Erev Pesach;
But when do they agree that one may start?
They do not agree about Erev Shabbos - they argue about this!
Answer: They agree that one may start before nine hours.