A gentler way to chase electrical sockets
Been in this position many times. The problem, as you have discovered is the fragile nature of the mortar brickwork. The only way to stop it falling apart is to use tools that do not generate such a large percussive shock to the wall. One such device is a masonry saw such as a stihl. On the other hands if your brickwork is that fragile it is hardly something you should be leaving that way. Perhaps do the damage and re mortar it.