I can't see it, so I could be wrong, but when cables start to sound dull, either a cold solder joint or corrosion would be my first thought.
Capacitance and resistance naturally occurs when a wire has an electro-magnetic field running through it, even if the totals of each are very small. Metal wires have very low resistance and as they get longer, the capacitance grows as resistance goes lower. When it reaches the input of an amp, the input resistance (usually 470k to 1M) will interact with the capacitance of the cable and act as a low-pass filter, throwing highs to ground.
It's part of why I put "true bypass" in the other post. With a large enough pedal board filled with true bypass pedals, capacitance will be an issue. A buffer, or a pedal with one, is great for the last stage of a pedal board to kill capacitance up to that point and a buffer somewhere near the front, but after a fuzz, will keep the pedals sounding sharp if a person uses a long lead to the pedal board.
To answer your other question, two wires (one signal or hot, and the other ground) will be out of phase with each other. It will cancel electro-magnetic interference if they run parallel or are twisted around each other. It doesn't affect capacitance. A shielded cable does the same thing as a twisted pair and can be substituted in an amp or other audio device. Humbuckers work on the same principle, except, it's two signals going in opposite directions, instead of one being a grounded wire.
If a person steps on a shielded cable in the same spot enough times to break the shielding up a bit, it is more likely to have interference come into the wire. Same thing happens if the shielding was cut and soldered to the jack with many shield wires missing or making poor contact, because the ground potential will be weakened.
The last bit about all of this is: wires of any type should be as short and straight as possible. In an amp, it will create hum if wires curve, especially if they pass over a power supply circuit in a curve or at an angle other than 90 degrees. With a guitar cable, it could also hum. Not only is it about the length, but also about excess cable wound in a circle. If it is wrapped around a laid down, it creates its own magnetic field (a coil) and can act like an antenna. Place that near an electronic device, a power supply, a wi-fi hub, or a stray Christian/Classical radio station signal, and you'll notice the effect. (Those kinds of stations are generally low enough on the radio-band to be picked up by guitar equipment).