The walls are faced with coursed stones about 20/30cm deep and the space between is 0.6 metre of rubble infill. I was present on a friends job a few years ago when a few of the coursed stones fell followed by a small avalanche of infill. I think once the fierce jet gets behind a coursed stone it removes material from behind very quickly, more quickly than can easily be controlled.Why is a powered hammer needed if the stuff is cottage cheese ?
if it cannot withstand a power washer set on low/med, then the mortar needs to come out.
That is a very interesting looking tool, it may be another candidate.why not get a rotary carbide mortar cutter fitted in the angle grinder theres loads to choose from https://www.pipestock.com/masonry-mortar-router-bit