The issue of Bnei Ya'acov as "k'sherim" is hard to understand.
If we were to apply the laws of mamzerut as we know them (children of unions of Chaivei K'retut), then 2 of Ya'acov's children - Yosef & Binyamin - are mamzerim (2nd sister during life of first sister); if, on the other hand, we don't apply such standard, then what standard should be used to say that all were "k'sherim"
The answer to this question depends on which answer one gave to the question of how Yakov could have married two sisters. I'll give two examples.
If one answers that they "converted" before they married Yakov and were therefore "k'Katan she'Nolad" -- "like a newborn child," then Yosef and Binyamin would not be considered Mamzerim even according to our standards (after Matan Torah).
If one learns like the Ramban that the Mitzvos before Matan Torah did not have to be kept in Chutz la'Aretz, then the title "Kesheirim" means that they were Kesheirim because the Mitzvos did not have to be kept in Chutz la'Aretz. Just because the standards were different does not mean that we would not think to make a Gezeirah Shavah that just as the end result of Yakov's children is that they were Kesheirim, so too one might think that people have to have the title of "Kesheirim" to perform Yibum.
All the best,
Yaakov Montrose
With respect to answer #1, don't we also have the concept that those relationships that the idea of "katan shenolad" should not be applied to allow that which would be prohibited.
With respect to answer #2, one goes back to the issue - kesharim have to be based on some test - if we say there's no test, then how can we pronounce kosher;
this goes to the issue of "im lavan garti" and the two explanations of Rashi
garti = i never prospered and was always treated as a stranger; or
gati - with the play on words (in ivrit only) taryag mitzvot shamarti
yaakov, keeping taryag mitzvot, is obviously no more definable than "kesharim" without any standards of kasher.
Possibly, one should adopt the same standard to stories in Bereshit as the medrash applies to megillat ruth "ein bo tumah v'thara, issur v' heter" and is there solely to teach midot.