Hi Sara,
I realise your original post is nearly a year old, but I am also researching this line. I don't have Jan's parents, but I he and Frantesha's daughter, Marie Kremenak is on my maternal side. Marie married Frantiske Zbanek and their daughter Josephine Zbanek married my mother's grandfather, Austin Ward Shontz I (there are 4 of them, so careful with that). Austin and Josephine had my great grandfather, Austin Ward Shontz II (went by Ward) who married Gertrude Scholtfeldt and they had my grandmother Sandra Shontz. Most of the Zbanek, Kremenek and Shontz line are buried in the Czech National Cemetary in Tama, Iowa (except A. Ward II and Gertrude who are buried in DeWitt, IA).
Not sure if that helps you in any way but thought I'd pass it along. If you have found anything about Jan or Frantesha, please let me know!
angie