Corrected entry: The Mayor, near phobic about germs, doesn't even react when Faith picks up a cookie in his office, takes a bite out of it and puts it back in the pile. Someone as obsessive as him would surely hate the thought of half-eaten cookies being mixed with the fresh ones. We know he plans to keep the cookies since he covers them over with clingfilm later in the scene.
Corrected entry: If even Faith can instantly realise that the gang would be willing to trade the Box of Gavrok for Willow's life, we can assume Willow knows it too. Why, then, when she escapes from the locked room in Town Hall, does she hang around to read up on the Books of Ascension? If she had escaped they could have destroyed the box, and the Mayor's plan would have been stopped, whether he had the books or not.
Correction: Willow knows from the gang's reconnaissance that all the entrances and exits from the building are guarded, but there are few if any guards within the building itself. If she tried to escape she would certainly be caught. Knowing that her friends would try to rescue her even if they weren't willing to sacrifice the box, she did some snooping on the off-chance that it would prove useful.
Correction: This is to show how he feels about Faith.