1. I have no idea, and probably nobody knows, except German Einsatzgruppen.
2. 1944-1946 and 1955-1959.
3. Probably very low. Jews preferred marrying within their own community, and the same applied to Poles. Defying the cultural and religious pressure was always difficult, and even today intercultural/interfaith relationships face obstacles. Consider, for example, this Rabbi's response to a question regarding an interfaith marriage:
"For Jews, "marrying within the faith" isn't a cultural preference or prejudice. Rather, it is one the commandments G-d gave us at Mount Sinai. A Jew who marries a
non-Jew transgresses a Torah prohibition. The practice of not "intermarrying" is in fact one of the oldest features of Judaism (...) We were chosen as a permanent protest group against idolatry and immorality. Intermarriage is therefore antithetical to the Jewish purpose and to the Jewish identity."
4. Related to 1. In this case, the personnel files of Urząd Bezpieczeństwa (communist secret police) may hold the largest part of the answer.