Is there a safe way to rewire original sconces?
Your pictures don't show any wire coming out of the wall at the sconce and the wires in the central box look like they're cut short with no slack to pull out of the box to work.  Knob &amp; tube is normally attached to the knobs inside the wall and run as separate discrete wires so you cannot pull new Romex &quot;through&quot; the old wiring.  Expect to run new wire from an accessible connection point all the way to the sconces, including installing new boxes and cover plates to attach the sconces to (yes, that means cutting holes, pulling wire, and plaster work).
I'm not sure what type of wire would be suitable to run through the fireplace surround due to heat from the fire. I would try to locate the first stud on either side of the fireplace, then run new Romex in that stud bay, installing boxes to relocate the sconces outside those studs, then patch the holes where they used to be.  Relocating the sconces would apply whether you're running 120v or low voltage.