1) I think you have to say that we do not suspect the brothers of deliberately marrying off their sister to somebody who they think she will not want to remain with and will end up being Mema'enes. Rather, we say that the brothers look for the best husband they can find for their sister and hope they will live happily together until 120. If it does not work out, and she decides to do Mi'un, we say they tried their best and now Rav Sheshes asks his question about whether it is considered as if nothing ever happened when she comes back to her father's house after Mi'un, and she still gets Mezonos, or do we say that if she leaves her father's house once, she can never return and receive Mezonos.

2) If we do suspect the brothers of deliberately marrying her off to a bad guy so that she will do Mi'un and lose her Mezonos, the brothers should realize that it could backfire on them. She might like the husband and remain with him forever and then the brothers will always hate their brother-in-law. So this is a reason to assume that the brothers will marry her off only to somebody they like.

3) I later saw that, actually, the question does not start: The Mishnah in Yevamos (107b) states that if the mother or brothers married her off against her will, she does not need to do Mi'un. So she would not be called a Mema'enes by our Sugya.

Kol Tuv,

Dovid Bloom