Once I create and dry my gumpaste shapes, will they survive being on buttercream and/or in the refrigerator overnight? Do I need to wait to adhere gumpaste shapes until serving?

I once had dried fondant shapes collapse while sitting in buttercream and I don't want to make the same mistake with gumpaste.

Anywhere there is moisture or condensation, the dried GP will absorb said moisture and if touched is more likely to break. It depends on the size of your GP object whether things will just break off due to gravity when its weak and moist. But small-medium flowers are usually okay. IMO: If you can, place any GP accents prior to delivery/pickup. Just to be on the safe side!

I agree. I have had a few pieces go limp on me because of moisture. Put them on right before delivery.