The Gemara concludes that R' Eliezer doesn't necessary hold that Maamar acquires the Yevama completely. (In the second answer) The reason he holds that he can annuls the Yevama's oath is because we refer to a case where the time that he needs to do Yibum arrived. Although he didn't do it yet, Beis Din obligates him to feed the Yevama. Since she's receiving her support from the Yuvam, when she makes an oath, she relies on the Yuvam to approve it, so it was only made on condition that the Yuvam consents.
Tosfos says that R' Yehusha that argues (and says that anytime there is more than one Yuvam no one can annul her oath) holds even if one gave Maamer, since the other brother can ruin it for him by giving a Get or having relations with her, the Yevama is not confident that he'll eventually marrying her, so she doesn't make her oath on condition that the Yuvam consents.
R' Akiva (that holds that a Yuvam never can annul an oath) we don't necessary have to explain him like Rashi, that he holds there is no Zika so he's a complete stranger to her since there is no connection. We can says there is really a connection (Zika.) However, since the connection is a lot weaker than an actual wife (as proof, we see that if someone else has relations with the Yevama there is no death penalty) therefore he's not enough of a husband that the Parsha of Hafaras Nedarim should apply to him
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