You guys are making this way too hard and toxic. If none of you have experienced the dreaded dowel pin leak in a 1st gen, then you may not have seen how easy it is to take off. I had a 1st gen with the leak and it soaked the bottom of the car, front to back, on the driver's side. That side had softened up the coating to where it could easily be scraped off. What was left after scraping came off with paint thinner and a scotch pad.
The untouched side came off nearly as easily, after it was softened with paint thinner, after scraping any loose parts off. Hardest part is getting the paint thinner to stay on the coating long enough to soak in and soften.
Mineral sprits works too, get the low odor type of either. Since the undercoating is bitumen based, any mineral solvent or petroleum based liquid, (gas, diesel, motor oil), will soften the coating. This method works well on fuel tanks too.