My theory about Kai is that he is something else from what we saw on the show so far.And he was imprisoned there by some powerful supernatural,most likely a powerful witch.And somehow Grams knew about his prison and was the only way she could save Bonnie by sending her there.
And I do like your idea of him being the only one who might get out of there.It will be a nice twist in the SL...and of course it will mean they will be stuck there for a longer time which is more than fine by me.
