Well the boy is the ruler so Littlefinger either kills him or manipulates him which should be easy since that kid is nuts. He probably plans to inherit the castle and marry Sansa.
Indeed. Oberyn has this aura of a total badass. An aura of a guy that doesn't even bother with people because he knows he can destroy them whenever he wants.
But I think that the fight will be epic and not lopsided at all.