Kerstetter Family Genealogy Forum
I have seen Kerstetter spelled all the the ways that you mentioned, except Castle. I have been led to believe that the spelling depended upon who interviewed the immigrant famillies. Many of those who did the interviews spelled the names phonetically. In addition many of those being interviewed did not speak Engish very well and thus there was plenty of room for misunderstanding and creative spelling. The US Census interviewers also were creative in their spelling of names, and this also led to a multitude of spellings of the same last name.
Good luck in your research.