I also wanted to come up with uses for those bits of the adventure that don't seem to go anywhere, by tying them into one of the greater plots. For example, the ghost in the mine adventure might possess someone and escape into the town, where it will....do what, exactly? It's never explained. Ditto for the prison, it's a lot of detail just to have a callback to Legacy of the Crystal Shard.
I didn't know where the prison was from, but me reading through that section got a strong "Nope, not going to include this one when I run the adventure." It has almost nothing to do with anything, and actually undercuts the rest of the adventure because despite the rest of the towns being at the edge of desperation, the prison seems just fine.
I haven't fully decided what I'm doing with the Duergar yet. One thing that I know that I'll be doing is changing the Dragon to leave after they're in the fortress itself and at least have had an opportunity to see the map of its path along with getting confirmation of its mission. Plus, I think it works more dramatically to have "We have intruders in the fortress, send the dragon to destroy the town NOW" as opposed to "What an amazing coincidence that the dragon starts just as you get there". As it is written, the PCs don't know that it's headed towards the Ten Towns to destroy them, and even if they come to that conclusion there's nothing remotely logical about why the PCs should try to catch it as it moves so much faster than they can. I imagine a lot of PCs would be thinking "Maybe there's something in the fortress to stop the dragon remotely" like killing the spellcaster controlling it.
I'm also thinking about doing a little bit more with the devil influence where they might try to work with the PCs. Right now, I really hate his motivation which is simply "Utterly destroy the Ten Towns because I'm evil", which is really lame. You brought your people out of the Underdark to this fortress and committed their efforts to collecting Chardalyn just so you can destroy some podunk settlements that in no way affect you? Besides, destroying the Ten Towns doesn't seem like a very Devilish thing to do. So, I'm thinking about having it where Asmodeus was encouraging him to go through these plans to
conquer the Ten Towns and rule them with an iron fist, but the exposure to all of that Chardalyn has driven the Duergar king insane and warped the plan, which has led him into more destructive goals. Or, I might just leave it at him intending to conquer the Ten Towns with the dragon using strategic strikes and the PCs invading his fortress forces his hand to where he sets the dragon setting to "Raze All" instead. Either way, the usage of the dragon seems like it's a
perfect element to limit Short and Long Rests when exploring the fortress, so I'm not sure why it's not used to that effect.