Has anyone experienced scorching or burning of cork heatbed insulation?
The real fire risks with 3d printers are electrical in nature. Lets say your wires come undone and happen to come next to the other lead. If it archs and happened to be in just the right position. Pretty unlikely. Once I did a bad solder job and when I was working on my printer. The wires literally burst into flame in my lap. No damage done (other than needing a new board). That said we often see people who have their boards catch fire in the flashforge owner groups. Weak solder joints, and over all bad quality.

Do you really have something to worry about with Cork? No. A series of unfortunate events would have to happen, and more likely your board will cause a fire. That said I would look into adding a layer of aluminum and reflect some of that heat back up. 

If you are that worried I would just remove the sheet. Unless you are having issues calibrating you heated build plate / PIDs there is no reason to use insulation. I only use it on printers that have a hard time hitting ABS temps of 100c. Even then I only use tin foil and try to trap the air underneath. 

Just a side note. If you are having heated build plate issues maybe what you really need is a heat chamber.