WD-40 won't hurt the belts in the least. Modern belts and hoses are extremely oil resistant. They are designed for interaction with the normal automotive chemicals used in an engine compartment. There are a handful of exceptions, such as diesel fuel, which can eat up a standard composition hose. But WD-40 won't harm them.
But belt dressings won't help them much either. The one or two times I used them (on pre-1970 vehicles with v-belts), all it did was make a big mess.
If you're very cheap, flip the belt's travel direction and see if that buys some time. Otherwise, replace the offending belt. Clean the pulleys thoroughly in any event. If a new belt starts getting noisy sooner than you like, then you need to take a look at the pulleys for misalignment or wear issues. The newer drive pulleys and idlers are more prone to wear-related irregularities than the older ones were. I've seen serpentine pulleys get unbearably noisy after the coatings wore off the drive surface.
The bearings on modern idlers and pulleys are typically sealed units. If the seals are not damaged, you can usually use common engine cleaners with confidence.