In a medieval European context, Jehan is indeed one of the many forms of John
However, it has a separate origin as a Muslim name, going back to a Persian world meaning "world", though it is also used in Arabic speaking countries like Egypt. Jehan Sadat is the widow of the former President of Egypt, Anwar
Not having read Inkspell,
, I don't know whether a medieval European or a Muslim origin of the name is more appropriate in that case.