This is a clarification of the "up-conversion" questions I asked recently. According to the many sources on the internet many Orthodox rabbis hold the general presumption that declared and observant but un-papered Jews cannot be given the benefit of the doubt that they aren't mamsers, and so an Orthodox Jew should not ever date one. Is this assessment correct?
I've never in my life heard of any rabbi, no matter "how Orthodox" he is, asking a regular person who did not grow up orthodox to show papers to prove he's Jewish - as long as he claims his mother is Jewish. There is a Tosafoth in Tractate Yevamoth that states that anyone appearing before a rabbinical court and claiming to be Jewish is believed absent any other evidence. If the person in question is asked about his past and indicates something that raises questions about his lineage, then before he is married he might be asked to provide some kind of proof, depending on the rabbi. Like if it's ascertained that his mother was a non-orthodox convert, or he's the child of a second marriage, which could indicate a mamzerut issue. I also heard of someone in a charedi community whose parents immigrated from the Soviet Union who received advice from his Rabbi to obtain documents showing his parents were Jewish, to eliminate any potential concerns before he got married.
No. It is completely normal for frum-from-birth Jews to date and marry ba'alei teshuvah (aside, perhaps, from particularly insular charedi groups). The safek mamzer issue arises in very specific and rare circumstances, such as a Jew who was given up for adoption under certain conditions.
But in practice, Orthodox rabbis today go out of their way to rule that a person (even with a safek) is fit for anyone to marry. The only real concern is with someone whose mother had been divorced or separated prior to his or her birth, because there might have been a halachic marriage and then no halachic divorce.
More generally, the law is that an undetected mamzer is not a mamzer. For that reason there is a tendency not to inquire deeply into someone's past and investigating the timing and validity of divorces and such.