The "reusable rivet" is called a "cleco" or a "wedgelock", there was a discussion on them recently in the "Tricks of the Trade" section. There is a spot weld drill for removing spotwelded parts, it drills away the weld in 1 layer of the material. I don't know of a drill type spot welder. The original spot welds are made by contacting the oposite sides of the assembled materials, often impractical with a home shop size portable spot welder like the Miller/Hobart or copies of it. One sided spot welds can be made with MIG, but they will leave a little blob of weld metal. There are tools that use a covered welding rod and a home size welder to weld sheetmetal, they magnetically retract the rod when it touches the work. Marquett sold them for years, Eastwood still sells something similar, I never used one. There are stud weld guns for dent pulling, there may be spot weld atachments for them, I am not sure.