Only 3 qualities?
> There are three qualities and only three qualities of a cable that affect its transmission of an AC signal:
That is wrong.
An obvious omission is shielding.
Another omission is self-inductance.
Conductance in the insulator.
Impedance mismatch between source, destination and transmission line.
Non-linearity in the conductor. (I believe this one could explain why some systems sound better after they have been used for a few weeks after being plugged up.)
And that is just off the top of my head.
However, you seem to be only thinking of speaker cable, and only the final one on my list is really significant.
Typical high quality domestic speakers are a difficult load for an amplifier, capacitance and inductance to deal with, plus moving cones that need their momentum controlled very quickly.
The biggest issue is resistance, and the two big issues are the thickness of the cable and the quality of the connections..
I believe the reason multi-strand cable is usually better, is due to it making a better connection to the plug or post. (as you say, skin effect is a bit irrelevant at audio frequencies)
I have found that using banana plugs seems better than just the binding posts. (The type where two grub screws clamp down on the cable.) And I got a noticeable improvement with locking banana plugs that splay the pin to make better contact. (Downside is when you have a builder who can't work out how to unlock them.) £8 a pair from Amazon. (Or £80 if you go for rhodium and snake oil plated).
Good engineering does make a difference to sound, however snake oil is very prevalent.
For my speakers I purchased 14 awg ofc cable and biwired them, each of those with two cables in parallel. using the locking banana plugs.
For interconnects I usually go for THX ultra certified cables.