Advice for removing rear vision mount from c12 2013 pulsar

[Billj]

 Hello. I need to remove the original rear vision mirror mount from my nissan pulsar c12 2013 model as the mirror has broken off the mount and I need to replace it with a universal replacement and a glue strip. The mount is glued on and not clipped like the modern mirrors. I have tried breaking the seal gently with a screwdriver and blade but it has damaged the glass on the underside of windscreen. The cracks have not reached the upper surface. I have now tried using string to cut into the seal but i cannot cut right through the seal and it is shearing on glass fragments. I'm wary of using the hairdryer to soften the glue as it may cause more cracks. Any suggestions please? I had thought of using solvent but not sure which is best for this older adhesive.

My options are: 

Fishing line/piano wire to saw through the seal

Solvent

Heat

Go to nissan/rear mirror replacement shop

Fix glass

Remove old mount or install new mirror above