I see what you mean by lack of neutral. The one along the fuse blocks doesnt seem to have a purpose, it looks like just the load side of fuse hots are running to the fuse holders and the 3 conductor neutral isnt connected to the knob and tube neutrals. This means you may have to do some digging, possibly the main knob and tube neutral is in the vicinity if you can open up the wall a little more.
Safest option is to find the where the first device is on these circuits and try to refeed with new 14/2 or talk the owner into a rewire - which may not be that bad if it is all original framing and it is not some mansion. Just abandon the old wiring, devices etc and wire as new.There are probably not many of the old devices on the original knob and tube.
