2. He doesn't fit in with the other two ghosts (agewise), and since he doesn't fit in, his teenage angst might kick in, and he'll get mad and kill the other two ghosts for not accepting him.
3. Impossible to make connection without having seen the prequels. Lucas is practically forcing the prequels down our throats.
4. Sebastian looked like a kind, fatherly figure to Luke when they looked at each other. There's no father/son moment with Haydakin.