That whole section is a little tough, because it's a sandbox, but there are parts that work better as in a narrative sequence.
Why do they want to head to Mantol-Derith? Are they just trying to hightail it back to the surface? I’d allow them to make that choice, but maybe one of the NPCs asks them if they could go by way of Gracklstugh? Or once they get there, some hook takes them back to one of the other cities?
Another option, which would require more work, would be to transport the events of one of the cities over to Mantol-Derith (but then you’d also have to figure out what’s going on in the original city).