They didn't clarify how ayan received the crown too....
Since KJ is a post, one has to be chosen if it's vacant...
One candidate is automatically chosen who killed KJ....
Others can challenge him....For becoming KJ...as jhumru did or ayan...and may be one of them need to prove himself strongest to become kj by either defeating or killing others....
Crown can be empowered by doing 3 sins..ayan empowered it finally sacrificing himself....and passed crown to roshni....that means one can sacrifice kj post by own wish. That can be concluded only from what is shown.