Well, I don't have time for pictures right now, but I CAN report that after a LOT of time sanding, dropping down to #60 grit sandpaper, I was able to clear the lenses up finally on the Impala. Had to laugh because once the bottom half was clear of all that crap, I could see some melted yellow material sitting inside the headlight as the very bottom edge ... looked like a piece of the amber reflector, but I couldn't see any missing. Oh well! It took a LONG time! I used the 60 grit to grind it down really good and achieved the milky haze that the 3M instructions made reference to. After that, I went up to 220 grit for a while, then got back to the 3M disks, using the 800 grit, and then the wet sanding with the 3000 grit wheel. Then, I had some Polishing Compound I had initially bought to see if just using that would fix the problem (not!), so I polished up all the lenses (Impala and Taurus) using that. Then finally finished up with the 3M supplied Rubbing Compound, which really didn't make much difference at that point.
I have to say that the Taurus headlights came out better. I didn't have to drop down to the 60 grit, but I did use some 220 on them. They are clear now, and look fairly close to new, I'd say. The Impala's got scratched up a bit by the 60 grit, and I wasn't able to buff all that out with the higher grit paper. Nonetheless, the Impala headlights are now clear, although you wouldn't mistake them for brand new. The process worked, and it was a good experiment. I just think the Chevy Impala headlights were poorer quality than the Taurus. The Taurus is 5 years old, subjected to the same conditions, driven 20,000 more miles (226k vs 202 k), yet the Impala headlights went bad like a double-pane window that had leaked all its gas out. Probably not much I couldv'e done about it.