The flying creatures are fungus gnats and they are indicative of soil that is kept too moist. My guess is that most of the roots may have already rotted.Scrape off all loose soil on the surface of the rootball. Move the jade to a sunny location. Allow the soil to dry out about halfway down into the pot before watering. Sprinkle some sand on the soil surface to help eraidicate the fungus gnat larvae that live in the soil.
It is the wet soil that is killing your plant, not the fungus gnats.