Question

Hello Honorable Rabbi, it is written in Yevamot 88a: Rabbi Zeira said: Because you were stringent with her at the end, you were lenient with her at the beginning. Then let us not be stringent and not be lenient! Because of preventing her from becoming an agunah, the Rabbis were lenient with her.
I understood that this is not really an operative principle that can be applied, but rather that the Sages made the following consideration: to be lenient with her (to allow her to marry with the authorization of the religious court on the basis of the testimony of a single witness) because of agunah concerns, and to bear the consequences if it turns out that the husband is in fact alive.
But Rashba on Gittin 3a writes (I underlined the part I am asking about):
Since the Master said: In whose presence does he give it to her? Rabbi Yohanan and Rabbi Hanina disputed it; one said in the presence of two, and one said in the presence of three. From the outset he examines things carefully and does not come to undermine himself. That is to say, even if the husband comes and protests, we pay no attention to him, and so too Rashi explained. However, the language of the Gemara does not imply this; rather, it implies that he himself does not come to undermine himself, but if he does come and protests, his protest is a valid protest. And this also seems implied from the fact that it says: if the husband comes and protests, he disqualifies it. And that which we say later (Gittin 6b), “And if you did so, you would help, for if the husband comes and protests, we pay no attention to him”—perhaps there it is specifically with one who brings a bill of divorce from the Land of Israel, and the reason is that he did not need to say it; and since he did not need to say “It was written in my presence” we elevate his status and it is effective, since the validation of documents is rabbinic. However, we certainly learn this from our Mishnah, which teaches: one who brings a bill of divorce within the Land of Israel does not need to say, “It was written in my presence and signed in my presence,” and if there are objections to it, it must be validated by its signatories. For since it teaches this here only regarding one who brings it from the Land of Israel, and does not teach this regarding one who brings it from overseas, it implies that specifically in the case of one who brings it from overseas, because you were lenient with her at the beginning, you are lenient with her at the end—so that if he comes and protests, his protest is void. And this is stated explicitly in the Jerusalem Talmud (chapter 1, halakhah 1), where it says: Rabbi Yosi said: The stringency that you imposed on him at the beginning—that he must say, “It was written in my presence and signed in my presence”—you made lenient for her at the end, so that if he comes and protests, his protest is void. Therefore it seems to me that when it says, “the husband comes and protests and disqualifies it,” this is what it means: he comes and protests and says that it is invalid and forged, and he thereby casts aspersions on it; and it turns out that he is allowing her to marry with a bill of divorce upon which aspersions have been cast, and there is only one witness removing it from suspicion. And it answers that there is not even concern because of aspersion, since he would not come to undermine himself. That is how it appears to me.
 
That is, Rashba understands that there is some ability for the Sages to play with the law like a zero-sum game, such that one can be lenient with her at the end if one is stringent at the beginning. That is the parallel principle to what Rabbi Zeira says. What is the logic of saying such a thing?
 
This question is a bit related to the previous question I asked, but it is still possible to look at them as separate questions. Maybe it still would have been better to ask them as one question; apologies in advance!