I'd vary the Troll King's stats every time the PCs fight him. Just bumping up his level is lame. Give him something
different he can do each time they face him.
Here are some additional encounters. I highly, highly recommend the troll ship one for the town.
That looks fun.
This article might also be interesting to try and mesh with the general story (plus the Troll witch is neat).
If you use the encounter with the Lamia and the four hook horrors, my suggestion: have the Lamia come
peeling out of the cave, shrieking for help, huddling against the PCs. Have her direct them towards the cave, telling her there are MONSTERS in there, and they have (someone important). She needs their HELP to get them back! And as soon as the PCs get in the cave, she uses Pacifying burst first. Then the Hook Horrors descend upon them.
The adventure also takes you into the Feywild, presents you a castle... and doesn't really utilize the castle at all. I certainly would have.
Also, there are two instances where you can easily drop seeds about P2. The two drow in the Troll King's chamber can be from the Drow Enclave. The two necromancers (see the first link) fit right in, too. Maybe they are there hunting the two drow who were meeting with the Troll king.