I pretty much assume this whole thread is one big spoiler, so that's unnecessary to bring up.
Just out of curiosity, why would you skip H3? I've read all the adventures so far (but played or run none of them), and the Pyramid of Shadows was the coolest, IMO, with Demon Queen's Enclave a close second.
I agree that the adventure, by itself, is actually rather good in that '1e wacky' sense. However, it's hard as hell to tie it to the previous adventures, story wise. In addition, I generally dislike the notion of "You're now in a dungeon. That will take you 3 PC levels to exit."
One way would be to take the dead prince in Trollhaunt and make him a known NPC earlier on. He seems to have friends who helped him raid the Trollhaunt. In the beginning of the Troll Haunt, he's already dead. This could be a pretty powerful moment if the party knew him and liked him. It would be a seed all of its own.
Yeah. The hairy part is
where to stick it, and who the Prince is. I want to avoid the Ye Olde "Here's a real powerful NPC adventurer. You, puny adventurers, behold him. he will save the day. You, go kill rats." So the How and Where the Barony of Therund relates to the rest of the area is the issue.
I could just drop Fallcrest into the Barony. My plan is to start the adventure with all the PCs in Fallcrest, the PCs all happen to be going to Winterhaven at the same time on a caravan (since the kobolds have been disrupting trade, travel up there is being made in a large group). 2 PCs were mentored by the mentor. 2 are seeking the cult. And 2 are traveling to Winterhaven to find a girl:
Her mother is a rich aristocrat, relative to Padriag. The Aristocrat's daughter was visiting Padriag's daughter, and from what reports the aristocrat has received, her daughter has gone missing! She is headed up to Winterhaven to personally seek investigations into her daughter's disappearance, and the PCs are her guards. (Padriag thinks she ran away with a boy, but in truth, the daughter joined Kalarel's cult.)