Good to have you back. Stay healthy.
Good to see you are still around @Clueless33, hope you can keep your health in check…
Thanks Dave. I appreciate the love.
This is surprising to me. I have yet to encounter an adhesive on glass that could not be removed. This includes the state inspection stickers that are notoriously difficult to remove. Always adhesive left over, some soft, some rock hard after decades. Even razor blades can’t remove all of it. I use mineral spirits and it takes some time on the really hard residues but it always comes off.
I know the pros have special film removal solutions like ATR etc.
I wonder if it is not actually adhesive but some type of etching that has occurred. Hard to believe because most films are applied using soapy distilled water or other inert solutions but perhaps they used some unfiltered, hard water or slightly acidic that affected the glass? Would love to see a picture of the residue…
I’d follow @TwinTurbo ’s advice on a small area. If you can clean one square inch then you or someone you pay can clean the whole thing.
What’s in ATR Solution? None of your business. They have a SDS on their website, but none of the four ingredients that they list are identified. I’m not going to make it, just curious.
One of my pet peeves although I can certainly understand why they do this. One of the best ways to give away trade secrets is to apply for a patent… Notice it says ‘patent pending’ and they developed it 20 years ago 
Surprisingly, many nasty chemicals are listed as animal carcinogens but not human because there is insufficient testing to confirm it. Some of the more common chemicals used; MEK, Xylene, Toluene are pretty nasty stuff. But these are insoluble in water, which this product indicates it is soluble in water. So, looking for similar products claiming to be safe and soluble in water, these seem to come up:
Mineral spirits shows up in many formulations of adhesive removers. I’ll stick with that 