Were the Amoraim Orthodox?

I do between a daf and three every week, and one thing that always catches my eye is the seemingly liberal orientation of the Amoraim. With the help of God, I'm going to start documenting some examples, not because I'm trying to prove any point, but because I expect good conversations will result.

Here's something from last Shabbos's learning:

The Mishna (Sota 41a) says the king read Hakhel on "Shmini" but the amoraim are astounded. Why? Shimi suggests the eighth day, but what the mishna means is the eighth year. So, with very little fanfare or chest-beating the amoraim amended the Mishna to read shminis. Here's the passage from the Babylonian Talmud:

Can it enter your mind [that the Mishnah means] shmini [which is masculine and suggests the eighth day of Sukkos]— Instead read it shminis [which is feminine and suggests the eighth year]

The implication is that there's some problem in the received text, a problem the amoraim don't hesitate to correct. Rather then (1) complain about the goyish and therefore unrealiable rules of grammar, or (2) insist that amending the text reduces the Tanaim to the status of primitives too stupid to notice an obvious error in their own text, or (3) announce that only a havadala oblivious moron would trust his own ability to read and understand Hebrew words, the amoraim (4) unceremoniously acknowledged the problem, and fixed it.

A 2ist century Orthodox Jew would not do this. He would say the mesorah is certainly correct, in all its minutia, and the problem is one of our own understanding, As you can see, this isn't how the Amoraim approached the text. Their default positions seems to be that the inherited tradition contained errors. I don't see how this can possibly be construed as an "Orthodox" approach to the mesorah, but perhaps I'm wrong. If you're aware of someone who sees this passage differently from how it appeared to me, please post a comment. I make no claims to omniscience or infallibility here.