Does the TMP novelization expand on what Scotty was doing there? Is there additional dialogue that's not in the film?
The one thing it adds is that Scott is talking with "a group at an engineering computer."
But I think the reason Scotty's there is clear enough from the timing of events in the film. Consider: Kirk tells Sonak that he's going to a meeting with Nogura that he expects to last mere minutes, that he intends to be on the Enterprise
following the meeting, and that Sonak is to beam aboard in an hour. Then, in the next scene, he materializes aboard the office complex, where Scotty complains about the new departure orders. It's about another six minutes before they reach the ship, then Kirk spends a couple of minutes on the bridge, then he goes to engineering to talk with Decker for a few minutes, and then the transporter accident happens when Sonak beams aboard.
That means that Kirk's arrival on the office complex was no more than about 40 minutes after his meeting with Nogura, unless that meeting took significantly longer than the three minutes Kirk estimated, which seems likely. So it stands to reason that when Kirk arrived in the office complex, Scotty had only gotten the revised departure orders less than half an hour before. He would've had to rework the whole refit schedule in an awful hurry, and where would you do such an administrative task from? The office complex. He probably had to meet with the other top people who had offices in the orbital complex and talk with them about how to fulfill the orders. When Kirk arrived, Scotty hadn't yet had time to get to the ship and begin implementing the new plan that he'd been hashing out in the offices.