Two Puzzling Baptisms

How could the Apostle Paul maintain in his first letter to the Christians in Corinth that all their ancestors were baptized into Moses at the Red Sea / exodus event (10:2), and how could he tolerate some of them having themselves baptized again on behalf of the dead (15:29)? Answers to these puzzling questions can be found in early Jewish sources now located both in Greek and Hebrew, all here translated.