Sorry I was not very clear, I was just referring to the walls proper. You can run each wall conduit back to the house or from wall to wall then the house, depends on what is convenient. Just be sure to size the conduct properly (larger is ok) for the number of wires required. Your local building inspector can tell you what he will accept. Add one wire for the earth ground, it must be connected to your panel. I would suggest you get some landscape wire and run one to each wall, then you can connect speakers to it (the purest s will complain) but it works. 5 amp at 8 ohms will give you about 200 watts. Remember to check the voltage drop in the wire so you can size it properly.

Your original contract specifies what he would do and what you would pay. If the scope of work changed OR the price changed, that would be specified in a Change Order that showed the new scope of work and the new price. It would have been signed by you. If it was me and he gave me a verbal estimate of an additional $300-400, I would pay it but you certainly wouldn't be required to do so without written authorization. Anything beyond that is crazy. It sounds like they are changing both the scope and the price. If they didn't look at the job carefully enough and missed the elevation change, they didn't do their job well enough. If they even considered building it NOT LEVEL, they are hacks.

“it fits the house era very well. Consider rebuilding the wall and fixing its deficiencies ... especially if it is drystracked now, as the demo would be a lot easier. It gives a certain charm that concrete will not.” I agree with this, though I don’t see a need for rebuilding the wall. Given a choice between a dry stack stone wall and concrete, I would pick the stone for all but the most starkly modern home. Concrete is a fairly boring material, and between the mess created to put in a concrete wall and the expense for something that isn’t an improvement to my eye, why go through the hassle if it is stable, functional, and suits the house. It is also more forgiving a material. A cracked concrete wall looks poorly cared for. The slanted walkway I would replace as finances allow for safety reasons. When you do that, consider making the replacement wider, at least 4’ or more. Skinny walkways look pinched and unwelcoming.

You need to use photographs to explain why things would be this way or that.