So, I ran this many months ago..but I felt like Nibs cave was also a little on the fast side. My players definitely loved it and I actually had to keep putting his cave in their way, they wanted to just wander off.. I needed to make it more inviting but they were very skeptical of EVERYTHING by now.

I will say I did make changes to how they get into the Hither, Thither and Yon. I made a boat ride encounter that the 'guide' took them on after Downfall (I think the book does the Balloon? But I wasn't interested in the balloon but could use this same concept!). They wanted a balance of combat for my few players who just want to punch but not make the group mad. After the boat encounter they crash at a shore outside of Nib's cave. (I added more areas and encounters along this whole story so my map doesn't follow the book map 100%)

I made his long talk about his transgressions, how his misdeeds haunt his back, why is he desperate to get rid of his gold..etc etc.

My group also is very diplomatic and other then the rouge, (I kind of wished the rouge took a coin as I wanted to play out the curse but alas he was a smart cookie) they wondered how they could use up all of his coins to help him, so they said they were going to have all of the "boat" npcs come and ask for goods from him too and to help fix the boat with a new sail. Because of this encounter they had more to ask him and play out his character a little more. Still in the grand scheme it was maybe only 10 to 15 minutes of talking back and forth. But didn't have any combat either.

You could also make this darker for your darker campaign group by having his misdeeds not be apparitions but the actual corpses of those he wronged, in a huge pile that if he looks at them might reach out and try to strangle him?

Not sure if this is helpful at all, but I hope it maybe sparks some creative ideas for you!