I recently had a similar problem with mirrors mounted in my second hand mobile home. I tackled one by removing the trim and used a hammer on it. I had a tough time, but found the mastic was only applied in small areas. I put strips of duct tape across the mirror at various points to reduce the crumbling and was able to get it down. I now believe the mastic is removable with the use of mineral spirits - but care must be taken as that is one highly toxic solvent. It will be a slow-going project, but should work. Maybe a catsup plastic squeeze bottle would work to apply close to the wall and the fishline sawing action used in combination will effect the desired results. Good luck!