In this third and final section of a teshuva about a child who may or may not have been a mamzer, Rav Moshe Feinstein considers the implications of the fact that the potential mamzer was someone who had spent several years learning in Yeshiva and possessed fine character traits. Can those be used as evidence to help clear up confusion over whether or not he had been conceived in sin?
While Rav Moshe is hesitant to make such a move in the case of a mamzer, he invokes a different case as a parallel. He had been asked years earlier about someone who was a talmid chacham and a God-fearing Jew whose mother was known to have not kept the laws of niddah. As such, while this young man was not technically excluded from marrying a Jewish girl, he would be considered a בן נדה, which would be at least a blemish in the world of shidduchim. However, since there is a statement from Chazal that says that a בן נדה will possess the trait of brazenness, and this young man clearly did not fit that bill, then Rav Moshe surmised that perhaps it was possible that he was conceived when his mother was not a niddah.
How can this be the case if the mother never kept the laws of niddah? Rav Moshe suggested that perhaps there was a time when she entered a body of water that would qualify as a kosher mikveh, such as by swimming in the ocean, and even though she would not have had intention to enter it as a mikveh, and even though she would not have kept some of the other practices associated with the laws of niddah, such as הפסק טהרה or שבעה נקיים, nevertheless on a basic level she would have entered a mikveh and thus perhaps this young man was born soon after that event and thus he can be considered to not be a בן נדה for lineage purposes.
With regard to the actual question in this teshuva, which concerned someone who was potentially a mamzer, who is forbidden to marry a Jew, Rav Moshe feels that given the other potential reasons given in the other sections of the teshuva (to be written up soon), the fact that he is also a Torah scholar and a God fearing Jew can be used as additional support for the fact that he is not a mamzer.