IMPORTANT! If you decide to use single-edge razor blades place a 1/16- 1/8 piece of cellophane tape on each side of the blade and scrape with the middle part "Gliding" on the tape, this prevents the edges from scratching the glass.

I have used this technique for over 20 years on a variety of surfaces including glass, ceramic, french polished wood and metal with no ill results. It is a well-known technique among repair/restoration Technicians to give a measure of control and prevent inadvertent scratches from the blade if not held at a low enough angle.
The old way was to round off the edges with a file and some still prefer this method. I would have used several shielded blades on this large project.
When giving advice assumptions must be made as to the skill level of the User, I do not believe this method caused the scratches even if the tape was torn. I will hesitate giving advice the next time and sorry for your issues with the glass.