When resetting the accounts old emails are removed from display but still in the .qmf folder but that's not of much use since they are removed.

You can try hotmail in mail for exchange; it syncs calendar just fine but i don't know if it syncs past entries. Maybe search if there is a way for the calendar entries; there was a way to extract notes from the notes app so since both use a database i'm sure there is a way to extract calendar entries too. I don't remember it well otherwise would point you to it

Your procedure is right. For removing the warning after enabling developer mode you need to type in terminal "disclaimer-cal remove View-openmode" then flash the openmode kernel and you are done