My question is: why does the Mishnah in Eiruvin is not a difficulty even before the resolution between Abaye and the Braita brought by Rav Sheshet?
That is: When the gemara asked Abaye from the Braita, the resolution was that the house close to the wall was secure or not secure.
But only then (i achi...) the gemara finds the Mishna in eiruvin a difficulty. But the secureness of the house does not say anything about the PLACE where the tefilin ARE right now.
What was the gemara thinking in the beggining for not asking from the Mishnah, and how does the answer after bringing the Braita poses a difficulty for Abaye from the Mishnah?
Joseph Kayeri, Sao Paulo
The Gemara knew all along that the Mishnah in Eruvin talks about a scenario where the Tefilin are not secure, as Rashi writes that he found them in the field, which is not secure.
In contrast, Abaye talks about a scenario where the person is wearing the Tefilin on his head, so the Tefilin are secure now, and they are also secure when he reaches his own house. This is why the Gemara did not find the Mishnah to be a difficulty at this stage.
It was only when Rav Huna, the son of Rav Ika, brought the Beraisa that we were forced to say, in the resolution, that a house might not be secure. This is when the house is near the city wall or near the Beis ha'Midrash. Before that resolution we thought that every house is secure, just like the person's own house is certainly secure.
When we now say that the Tefilin are not secure, we ask why does the person have to be wearing them on his head in order to be permitted to bring them into town? The Gemara asks that even if they are in the field, we learn from the Mishnah in Eruvin that he may bring them into town. This is why the Gemara brought only the Mishnah in Eruvin at this stage.
Wishing you very much success,