My history major friend and I have conflicting book about how descendants of Jews heritage were identified.
Main Question: IF 1/4 of person A's grandparents was Jewish, would A have been seen as a Jew?
No, A. would not have been seen as a Jew, but as an "Mischling 2. Grades" if we are just talking about one grandparent being Jewish. The case would only be different if A. was part of the Jewish religious community.
As an "Mischling 2. Grades" A. would not have been allowed to marry someone who was (at least) 3/4 Jewish and would have only been able to marry a "Mischling 1.Grades" (1/2 Jewish) with a special permission.
Source: Nürnberger Gesetze