1) The Oathbreaker must be a Planeswalker on its front side. Arlinn Kord could be your Oathbreaker. Jace, Vryn’s Prodigy could not.
2) Each Planeswalker has their own color identity, and therefore, their own Signature Spell. This means that you will have 4 cards in your Command Zone (and only 56 in your library). You may only cast the Signature Spell of the Oathbreaker currently on the battlefield and under your control.