Jk has already said that Mark Evans has no significance, and that she regrets giving him the surname of Evans (see her website under Rumours Answered). I would bet the Prince is a new character, maybe a foreign wizard, or a wizard from the past. Look for clues in the Hogwarts graveyard, which she says will come into the story in Book 6. Also, don't forget about all the stuff in the Dept of Mysteries. Harry still has the mirror. Did you notice only he and Luna can hear the voices behind the veil?