Sources for zera yisrael: R’ Haim Amsellem’s Sefer Zera Yisrael; Shut Mishpetei Uziel EH 13:5; R’ Tzvi Hirsch Kalischer, Drishat Tzion (don’t have the text al yad).
I’ve heard speculation that Elvis Presley and Abe Lincoln were descended from Melungeons, who supposedly have Jewish ancestry.