Fungus gnats live in peat and other decomposing organic matter, no way around that. I live in the gnat capital of the world and have given up on any kind of store-bought potting soil or mix for all but very big pots (because it is more lightweight) because I end up with gnat farms and rotting roots. By now, most of my small pots just have ground dirt. I can change it for free whenever I want - and ants have also quit building nurseries for their eggs in my pots.
There's no reason to use any chemicals to get rid of them, just find a BTi product that can soak in your watering can and then when you water, you'll be killing fungus gnats too. A well-known brand is Mosquito Dunks because it's primarily marketed to be used in fountains and bird baths to prevent mosquito larvae from being able to survive in those standing water venues.
If plants are so thirsty that dew or mist is making them perk up, they need water in the soil, not on the leaves. Just like any other plant, succulent plants can die but still look OK for a while as they slowly become desiccated or mushy, whichever is going to occur. It is easily possible for the roots of a Jade to become so dried and shriveled that they are dead. If that happens, there is no way to save it except by chopping off what is still viable as cuttings.
