I would not be so quick as to say that removing the sheetrock and dealing with the wall cavities is overkill. As a matter of fact, I don't think there will be a realistic choice, since all of the duct work and any wall that had any exposure to the room air, such as around an outlet or switch, fiberglass insulation, etc. Depending on the age and type of mechanical systems for air handling, there may be extensive ducting running up the walls. It may be easier to simply strip the sheetrock from the house, remove all duct work, deal with remaining mold with chemicals or surface abrasion, and then install new air handling, insulation, and sheetrock.