The reason that using the back of a pot for a ground connection is frowned upon, is that the threaded mount of the pot creates a connection between the pot shell and any shielding used in the control cavity. This creates multiple paths for current to flow to the ground connection at the jack. When you have multiple ground paths, each of those paths has a slightly different impedance, and that means that current flow will cause a slightly different voltage to appear across each of these paths. For the same reason, different signals should not share a common ground wire, because this will cause interference between the two signals. These voltage differences cause noise, fluctuating frequency response, and sometimes result in wierd things like having the audio portion of radio or TV signals come out of your speakers.

Grounded shielding is most effective when there is no current flowing in it. So the copper foil or conductive paint should not be connected to any signal path, except at one point, usually the jack. All ground points on pups, switches, and controls should be connected to the jack ground by individual wires that are not shared with any other part of the circuit.

That schematic in Post #4 shows the bridge ground wire connecting to the humbucker shield wire, and a ground connection from this common point. Not good. The humbucker shield wire, the bridge shield wire, and the ground connection to the Volume pot should all be separate wires connected to the shell of the jack.