As someone who works in the glass industry, I can tell you there is a general rule about this... if you can stick your fingernail in the scratch then it cannot be removed.
I have had to deal with this on many jobs where the cleanup crew (not the snails) ended up scratching 1/2" glass on handrails and such, and no matter how much $$$ and equipment we threw at it, in the end the glass would have to be replaced.