Dear Rabbi Chrysler,
Thanks a lot for the great response!
I have another question, but this time about 12b.
We see starting 6 lines down from the top of the page Rebbi Yehoshua ben Levi's proof to why we still give ma'aser rishon on shana hashlishis of the shmitta cycle. Then in the middle of the page (line begins Eliezer Ben Yaakov) we see Rebbi Eliezer Ben Yaakov give the same exact proof, but in a Baraisa. Why did RYB"L have to come and say the same thing that is already known in a Baraisa? Is RYB"L's proof simply there to support REB"Y with an amoraic statement?
Had the Tana'ic statement been contained in a Mishnah, the problem would have been more serious. Every Amora was expected to know all the Mishnahs by heart, and there could be no such thing as an Amora issuing a ruling that had already been taught in a Mishnah.
A Beraisa however is different. There are so many different collections of Beraisos (all transmitted orally), that it was simply impossible for any Amora to know them all. Consequently, it is quite common for the Gemara to cite the statement of an Amora, and to support it with a Beraisa (of which the Amora was unaware), and not vice-versa, as you suggest (as borne out by the words 'Tanya Nami Hachi').
The reason that the Gemara mentions the Amora in the first place is presumably because his statement was already made and duly recorded, when somebody in the Beis-Hamedrash introduced the Beraisa in support. So that is how it remained.
Be'Virchas Kol Tuv