Removing excess mortar/patch work

Any tips for removing excess mortar/masonry patch work from sand stone walls? We have an old old Ozark stone house that is in fabulous shape structurally that we are trying to preserve. We are building, essentially, a house over the existing house. We don't want to add any load or destroy any of the original rock walls so we are building a bigger house that surrounds the old house completely. As such, we don't need it to be insulated or anything, We want to expose the rock on the interior of the house and have removed all the non-original drywall that was added over the years to expose the framing and rock ...

... but ... the rock on the interior of the walls was never meant to be seen and therefore isn't "pretty". It has a lot of excess mortar smoodge  and a few places that were patched at some point in history. We'd like to smooth these rough places out but have no idea where to even start. Hammer and chisel? Some sort of grinder?

There are couple ways to go about it. The hammer and chisel that you mentioned is one way. Mortars can vary in quality, not just the base ingredients but mixing it too wet or too dry can result in a poor bond. I removed some tile on a job once and the tiles popped off way to easy. What happened was the installer mixed his thinset mortar too dry and it did not make a quality bond, which was great for me doing the demo. Mortars mixed properly can be extremely difficult to remove. Try using a cold chisel and hammer and if you place the chisel on the edge of the mortar blob where it meets the substrate and firmly strike sometimes the chunks will pop right off. Another method is the diamond grinder, which I've included a picture of. It's a 4 inch diamond wheel that goes on an angle grinder and makes quick work of removing even the hardest, best mortars and concretes. I mean it's like a hot knife in butter, but it makes a ton of very fine dust, and a high quality dust mask should be worn so silicates aren't inhaled. No one wants silicosis. I've been using mine to remove humps and bumps in concrete pads/slabs before setting tile.

The fastest and cheapest option is most probably pressurized water. We use this method whenever required. With right "gun" type you can do many things with a car washer. If you use point-tip (the piece that decides the shape of the water beam), it can cut concrete (with regular washer it might take time though), break (turn into sand) excess concrete, open holes etc. Line-tip gun , which is the most widely used, can clean excess mortar, clean tanks that have rigid walls. You can also clean plastic tanks if you have a distance of 4-5 ft (~2m), but I don't recommend that. There are also eye-shaped, star shaped versions that work better on cars. Generally speaking almost all car washers work at the same pressure level. So it does not necessarily mean that you need a big one to do some real work. Small ones do the exact job, but you might need to take a break every 30 mins. We have Karcher here in Europe, Bosch also has models too. Karcher K2 and up will do.
It is fast, no dust = clean and cheap.
Just a word of caution. Pressurized water beam might be really dangerous though. It cuts through concrete, flesh and cloths won't stand a chance. Don't let dogs and kids be free when working, they fall in love with water jets. For yourself wear eye and etc. protection!

For removing harder mortar, I use a pneumatic chisel.  A toothed chisel works well for quick removing.  A flat or curved (rondel) chisel works good for cleaner appearance.