How to remove "metal" tile adhesive from backsplash area?
The amount of time and labor involved in removing adhesive would be extensive. Even if you get the adhesive off, you will likely damage the surface of the underlying drywall.

It may be much more efficient and effective to remove the drywall and replace with new.

Use of a heat gun is questionable. Heat could crack the nearby marble if held near it too long. It also can char the surface of the existing drywall, making it less stable for the next finish.

Asbestos is unlikely in a project done in the 80s. However, if the house is older, there could be asbestos covered pipes behind the wall surface from before the old reno. If you open the wall to replace the drywall, just be careful not to disturb any coverings on pipes.