And they are arguing regarding this - ובהא קמיפלגי
Summary:
The גמרא assumed that the אמרי בי רב must be discussing a case of אייעד; otherwise what is the novelty of his ruling. We cannot refute the אמרי בי רב from the ברייתא either because רב תנא ופליג, or this ברייתא was not taught בי ר' חייא ור"א.
רב אשי establishes the משנה like the רבנן, and he queries thus - רב אשי כרבנן מוקי לה ובעי לה הכי
Summary:
It is preferable to establish the משנה according to a רבים (even if it requires some reinterpretation of the גמרא). Occasionally the גמרא inserts a [controversial] interpretation of a query which the original query did not assume.
In a place where it is impossible for her, unless, etc. - במקום שאי אפשר לה אלא אם כן וכולי
Summary:
This query is not discussing a case where it happened in the רה"ר. We can infer the meaning of one אמורא in the context in which it is mentioned in the גמרא.