I've never seen dethronistaion in my life.
There is a somewhat amusing (though not very reliable site) site called "googlefight."
You enter your two key words and ask google to do a comparison of how often they are used. Dethronement won, 6430 to 1. In fact, the only google hit for dethronistaion is this page. A plain google search showed 196,000 hits for dethronement, not the small number given by googlefight.
Student or Learner