Am I missing something, or is there not really any lead to encourage the PCs to go back to Sandpoint after defeating Mokmurian?
The PCs in my game were married to people in Sandpoint, by this point in the campaign. Or they had lovers in town. And they were pretty greedy and loved returning to the Rusty Dragon for the free food & rooms. Also, one of the characters had retired to help restore/run the Glassworks. So I imagine they would have come back anyway. Having said that, I took the rumors/research that is offered on page 233 and 248, and freely gave out that info when they went into the library to research Xin-Shalast and the Runeforge. This helped them to see that the Runeforge was important, and from the handout that you mentioned, they knew that someone in Sandpoint had a key to it.
Based upon the advice on page 232, I also allowed them to research "the traitor Xaliasa" from the handout. The ancient library with it's clockwork librarian wouldn't have any info on Xaliasa turning into the Scribbler, as that's new information. However, we know from module/book 5 that he was a powerful general in the old runelords' armies. So, not a big stretch to at least have the names of some generals and where they served. Of course, when they served, the entire area was landlocked. So it was fun to point that out, and have the players say, "but that's near Sandpoint, and that's not land-locked!"
By this point, I was also trying to make it very clear that the "lighthouse" in Sandpoint was a hellfire flume used by the runelords, which further makes it clear that Sandpoint was a strategic military location. All of this may send them back to see if they can get information or find out what Mockmurian had learned.
However, if all that falls through, don't forget that the module says that delays in returning = Father Zantus gets a Sending spell cast to tell them to return. So at least for this part of the game, they players don't need to be motivated, and don't need to be interested in the notes or the library. If they just blow it all of or go in the wrong direction, they'd have all of a single day to wander or waste time, and then poof, Zantus puts them on the right track.