When I got married, there was some issue over how my wife’s name would be written in the documents because her father is not Jewish. If memory serves me correctly, the rabbanim involved decided on using her maternal grandfather’s name. Some time later, it turned out that the name we were told was her maternal grandfather’s name was never actually his name. He passed away some years ago and was never given a formal Jewish name. Does this call into question any of the documents using the presumed name of the maternal grandfather?
Yes, the validity of the kesubah document is called into question, and a new kesubah document should be written up. The kesubah can be written with the mother’s name.
Sources: See Beis Shmuel, Even Ha-ezer 129, concerning the change of a father’s name on a get document, and the same laws apply to a kesubah document. Harei Besamim (tinyana 172) writes that for documents, where there is no concern for the person’s shame, the mother’s name should be used, citing numerous precedents for this.