How should I go about a partial (bottom 1/5) exterior wall stud replacement?
For a 1 time event like a broken waterheater or pipe failure I doubt the studs need to be replaced. A 3% Hydrogen peroxide works better than bleach and dosent stink. All wood has mold spores at the lumber mill we spray the wood after the finish cuts or it will look ugly in a few weeks. The fungicide is only a short term preventive measure, keeping the wood dry is the only way to prevent growth. Using a screwdriver push in on the wood If it is rotten the blade will go in with ease If just surface mold the screwdriver won't go very deep unleaded pushed hard.  If the wood is sound a 3% hydrogen peroxide and water solution is all that is needed to kill the surface mold once the area is dry replace the insulation and sheetrock.