The Gemara later ends up with "teuvta" on Rav. Why doesn't the Gemara
give the usual answer that Rav is also a Taana and can disagree with
The YAD MALACHI (#544) writes (from the Acharonim, and from the Ritva at the beginning of Maseches Eruvin, and from the Shitah Mekubetzes in Bava Metzia) that the Gemara only gives the answer of "Tana Hu u'Palig" when there is no other answer at all. It is considered to be a "forced" answer.
It could be that the Gemara could have answered here that Rav disagrees with the Beraisa, but since the Halachah is not like Rav in this case, the Gemara did not strain itself to give this forced answer.
The MELO HA'RO'IM relates to your question. He explains that when the
Gemara says that "Perhaps Raba acted upon Rav's ruling, even though Rav was refuted from a Beraisa, because he did not hear of it ('Lo Shemi'a Lei')," the Gemara really means that he did hear of the refute, but he was not convinced by it since Rav can argue with a Beraisa.
It should be pointed out that "Rava Tana Hu u'Palig" means only that Rav may argue with a Tanaic opinion, and thus if we find a Tana who argues with him, it does not mean that we are misinterpreting Rav (as would be the case with other Amora'im). However, since he is arguing with a Beraisa, the Halachah follows the Beraisa and not Rav.
According to Rav Chaim Soloveitchik z'l as mentioned by Rav Elcnonon Z'l, an Amora could dissagree with a Tanna because Amoraim and Tanaim learned everything baal peh. Therefore, although an Amora could dissagree with a Tanna, this happened rarely and not solely because this was a terutz of last reort.
The comment attributed to Rav Chaim z'l by Rav Elchonon is found in one of
his seforim on Kesuvos on the Daf qouted by you. Rav Moshe Soloveitchik z'l
is quoted by the Rav Z'L and by Rav Schacter that Tannaim and Amoraim both
learned baal peh and that this is the reason why Rav Ashi and Ravina are sof