Love the campaign background...
Love the story and the background. If you actually have players actively follow this complex a storyline, consider yourself lucky.
Here's what I would do...
The PC's fame spreads throghout the land as they overthrow the evil magistrate. It continues as the PC's do more and greater deeds, all of which put a wrench in the plans of the Crown Prince.
In the meantime, the younger borther to the prince gets married to the wackjob overzealous religous freak princess of the other tribe. With the mad princess urging him on, the younger prince declares himself rightful king.
In the meatime(2), some baddies get wind of Arselain and his hiding spot with the druids. They sack the druids home, kidnap Arselain, and he declares himself to the rightful king, even though he is nothing but a puppet of those who captured him. Over time, he sinks into despair and nearly drinks himself to death.
In the meantime(3), the queen is killed off by someone (lots of options).
In the meantime(4), the king goes COMPLETELY insane.
In the meantime(5), the Crown prince, with a foreign invader coming (his younger brother) and an internal civil war brewing (his son Arselain's forces), starts taxing the people to the breaking point and conscripting young men to join his army, all of which obviously causes much resentment. At this point, it seems as if the kingdom is going to fail.
In the meantime(6), a wise, well-known and repsected prophet (perhaps the magic man the PC's rescued earlier), declares the end of the kingdom is coming and only the children of the land can save everybody from ruin. The common people, heeding the prophets words, witnessing the great deeds the PC's and seeing no viable alternative, start to plead with the PC's to take control of the kingdom themselves. "All options are terrible," they plead, "and only true heroes with a conviction greater than the law can save the land."
Of course, this puts the PC's in a lousy, no win situation. On one hand, if they respect the law, somebody who does not have the best interest of the people take power. On the other hand, if they break the law and sieze power themselves, they are essentially performing a coup. I love putting the PCs in tough spots...
I bet the PC's would opt for the greater good and take over for themselves. Of course, they may come to realize the king's third child, princess Merisende, even with her sickness, is the most qualified of all and their coup can be an attempt to put her in power instead...
GL and I would love to hear how it turns out.