Lets see! Tricky situation you're in here. I'd recommend spending an point on Empathy to see if the King's top knights are still loyal to him.
Try get a count on how many knights Lothar has with him.
Tossing your lightsaber is a bad idea, just having it holstered and inactive should be enough to show your peaceful intentions.
Other then that your idea of convincing the King to hear you out is a solid one, you have high persuade which is good. Clearly Lothar is waiting for something before he betrays the King, I don't think he'd be willing to start a fight in front of him at this point, so you should be safe.
That's just how I'd handle the situation though, if Wentar were in your place.