1) Nope, but it's polite to give your local agency a notification call if you plan to set off large amounts so there's no mystery when the libs and whiny folks call them about "terror bomb explosion sounds" or whatever. Saves their time and yours.
2) Tannerite is an explosive and is inherently dangerous, but as long as you're clearing dry foliage and have good standoff to protect you, others and property from fragments you're good. 50lb and 100lb blasts can set off seismic sensors at power plants depending on your proximity to them, so see the suggestion above.
3) As long as the car is titled in his name and no fragments leave your property then no, it is legal to destroy your own vehicle.
4) Maybe check around with folks who own gravel pits? Those would be the safest area to make a car into confetti.
Note: Remember to take a video (HD please, too many potatocam videos on youtube) and pics when you do find a place!