Hi Legopart, if you can, don't use the shielded cables, they are heavier and have higher capacitance. And using shield as a signal or bias wire is not a good idea. It will work for sure, but you may get some noises and hums. Also please check the specifications for the insulation of the cable - it should be no less, than the bias voltage, otherwise you risk leakages between cables.
In this case only one cable is not have double insulation but it still insulated from the outside, it called shielded wire.
The shield wire used on my Et1000 as Bias cable (with same structure).
I decide to change it on my build model a little bit and set the -- minus as shielded one and the internal cable will use Red cable for ++Plus and Back for Bias voltage.
I think that it safety enough, regular headphones not having any issue to use one isolation layer, Stax headphones using only one layer for each of 3 wires for each side.
I know the electronics roll that the modern electricity cable must have dual insulation, but we still talks only about headphones and wiring for it.
another questions about the anty-static spray:
- Why the resistance is gone after the spray dries ? what to do to keep this resistance ?
- If I having some vintage headphones, do they need new re-coating on the diaphragm to work better (like old Stax old one)?
The flat ribbon cables are better with capacitance and easy to work with. I used 10 wires flat ribbon cable, skipping every second wire (not connected) , so you increase the insulation and decrease the capacitance at the same time. A standard stereo audio cable (like it looks from your picture) will also work, but I don't think the isolation voltage is more than 2-300 V.
The resistance of 1Mohm for a membrane is low, I don't know how you measure it, but of course once it's dry, you can't get reading anymore unless you have a special high resistance Ohm-meter. Don't worry - even you can't measure it, it is there - probably around 100 MOhm or more, which cannot be measured with a standard multi-meter.
my first measurment with my recently bought miniDSP Ears measurehead 
but now with the minidsp EARS i can make lot of test for example mylar tension differnt coatings and so on 
