R' Shlomo Zalman HaCohen Rapaport is profiled in History of the Jews in the Duchy of Mantua by Shlomo Simonsohn,Tel-Aviv University (Kiryat Sepher, Ltd.), 1977 Jerusalem: "Cohen-Rapa, Solomon called Zalman son of Menahem Rafael, rabbi in Mantua and Venice in the sixteenth century. He was a relative of Rabbi Meir Katzenellenbogen and was on of the rabbis of the Ashkenazi Yeshiva of Mantua. Cohen-Rapa belonged to the Tamari faction in the Tamari-Venturozzo divorce scandal. Some of his responsa and correspondence have been preserved in manuscript. He died during the plague of 1575-76, and Rabbi Samuel Judah Katzenellenbogen euologized him (in "Shneim Asar Derashoth", no. 11).,
If someone is able to read the eulogy by Rabbi Shmuel Yehuda Katzenellenbogen, MaHaShiks, MaHaRaM son #1 described above (in Rashi script) there may be a clue in those pages explaining how R' Shlomo Kohen Rapa is related to the Katzenellenbogen family and more particularly to: Rabbi Meir Katzenellenbogen, Maharam of Padua Here is the eulogy as a PDF: https://1drv.ms/b/s!AuwT-4qnkJLBn3X45Y9-s6c0sBEf?e=L8he7j