For the path that the party kills the dragon/princess without noticing the witch's identity, I imagine the disguised witch could lead the party to "the king's castle" to celebrate the end of "the terrible and evil dragon." The witch could lead them to a trap they planned with crazed wild animals or minions. For a more interesting, political route, the witch could try to get away from the party (maybe they go bathing in a creek to get the "dragon" blood off of her if the entire party happens to be of men who don't have prestigitation as a spell known, lie about needing to use the restroom and run away, or just by running away), gain a new disguise as a nameless travelling merchant, and enter the castle decrying the death of the princess by the hands of the party to enrage the king. After the king attempts to kill the party off (whether successful or otherwise), the witch can assume the disguise of the princess and exonerate the party, saying that it was the party that saved her from a dragon but lost track of her while going through the woods when she stopped to gather flowers/use the restroom/bathe/whatever. The party is celebrated (undeservedly) as heroes. If they are living and have figured out the witch's ploy, they'd have to admit to killing the princess to expose the witch. If throughout all of this the party remains clueless of the witch's disguises and lies, the witch becomes the queen when the king dies of an undetected poison. For this option, I'd advise the witch to mechanically be a Sorcerer with ranks in
Disguise and Bluff (though I don't know how you are building the NPCs). Your story's witch doesn't need to be a Witch to be a witch.
With the TPK, I think the party should realistically have a chance in escaping Paladins. Paladins aren't quick, sneaky, or skilled trackers (unless they are gest/tristalted too). Instead of trying to make a TPK inevitable, I would consider giving them something of a chance to escape the king's wrath. That avenue's Part 2 would lead to the party trying to assassinate the king, attempt bluffing innocence, or running away (Yay! Survival's tracking mechanics can see some use!).
As for the route where the princess is saved and the witch is dead, it could be a happily-ever-after ending when they get to the castle. Simple stories can be nice. (:
