*WHITE PAPER* The Sound of Music - How & Why the Speaker Cable Matters

Goertz offers their "RC network" aka zobels.

Of course to simplify all this just get some Canare 4s11 or other good copper speaker wire and stop obsessing. 

Can audio2design explain the change in responses as shown in Fig 3. and the close correlation with Zo, if it is not a transmission line effect?

All conductor pairs have capacitance and inductance so must have a characteristic impedance. There is no frequency component. Read All About Circuits, chapter 14 thoroughly and the two papers associated.

There are long transmission lines and short transmission lines.

Silversmith cables have Zo between 800 and 1800 ohms, depending upon the spacing.

Don't use conditionally stable amplifiers. 10nF is not a difficult load for a competently designed amplifier.

The measurements are of the voltage between the two black terminals at either end of the wire. The short circuit has the least voltage drop and is lower and the cables have a greater voltage due to resistance and are higher on the graph and the treble rise is dependant upon Zo. They are not inverted.

As I suggest, do the experiment your self before guessing.

@townshend-audio - What is the magnitude of the error voltage with respect to the voltage at the load/speaker in your measurements?

Transmission lines.

When do you have to bother?

Answer: long cables or high frequencies. You can completely ignore
transmission line effects if length ≪ u /frequency = wavelength.

Audio (< 20 kHz) never matters.
Computers (1 GHz) usually matters.
Radio/TV usually matters.