Window tinters use stainless steel blades because it apparently doesn't scratch glass.
unfortunately that is not true, the stainless steel blades will scratch the glass, you want the carbon blades. Also it depends on the type of glass, looking at the photo being up that high it would either be laminated glass or tempered/heat strengthened. If it is tempered pray that it is not the tempered side of the glass. If its not the tempered side or tempered glass you can use the razor blade on the glass, just use a slip solution and you'll be fine to take off excess tape/silicone.
To take the letters off I would use fishing line wrapped around wooden dowel (make a gareth) and use it to cut through the tape/silicone holding the letters on (use thicker line if the thin stuff breaks) then once all the letters were off I would use the razor blade to take off any excess tape/silicone. The whole thing shouldn't take longer than half hour to take down all the letters and get the glass cleaned up.
Chaz