What's better, one conductor or two conductors for an RCA interconnect?

Speaker by definition is balanced.  Noise currents induced in both wires cancel at the speakers, since there is no other return path.  If you question that, then perhaps you should read what Benchmark Media posted on their website in regarding to headphones being always balanced:
 

Headphone transducers respond to the voltage difference between the two wires that feed them. They have perfect rejection of common-mode interference because there is no path to ground or to any other conductor. In other words, there is no path for ground loops.

Headphone transducers are electrically isolated from everything other than the two wires that feed them. It doesn't matter if both conductors are driven differentially or if only one conductor is driven. The headphone transducer will reject common-mode noise.

The same happens when analog ground is floating on one side - no additional return path, hence inherently balanced connection with single ended output.  Analog ground is likely to be connected to chassis ground to reduce noise coupled from the chassis ground to circuitry, but it is often done with resistor of 100k or more.  It is because connecting analog ground to chassis on both ends directly or thru large capacitor creates perfect ground loop (earth ground - chassis - analog ground - interconnect - analog ground - chassis - earth ground).  That is why Benchmark amp doesn't even have single ended input.   This additional return path, possibly thru (two) 100k resistors creates some unbalance, but doesn't completely null usefulness of twisted pair in SE connection and that is why manufacturers use it.  For speaker wires it is no-brainer since it reduces noise, reduces inductance (important) and increases slightly capacitance (not-important).

As we continue about our qualifications, I assume that you've never designed any electronics?  As for me - I don't have PhD (only MSEE), meaning I'm still able to learn  :)    One more thing (Columbo) - it is "kHz", not KHZ.  I don't want to be unit police but I write it proper way automatically and it bothers me a little, when educated people don't pay any attention, sometimes using even erroneous terms like "watts rms"

As I clearly said, from the speaker standpoint, the speaker itself is balanced, however, if you are considering noise on the speaker cables induced back into the amplifier, it most definitely is not balanced because the impedance on either cable is not the same as the other. That should have been clear when I discussed a similar circuit such as a bridge or other similar sensor (or consider it a phono cartridge), that while not grounded itself, is potentially grounded at the phono stage end. Someone else on this thread, if I am not mistaken, makes a differential input phono stage?

If you have two AC connected pieces of equipment, then the ground on either side is never floating even if one piece of equipment has no ground connection due to parasitics. Now obviously those will be lower than if there is a direct ground connection. I was quite clear this would be dependent on implementation. 100K is pretty similar to the loop resistance of single ended RCA connection. I threw a question at some EEs. They said more likely the connection would be a capacitor, at least in the test equipment they develop(ed). I go back to my example of 10nF being 16K at 1kHz. This does null benefits of twisted pair and since the question was twisted pair or coax, it negates potential advantage of coax and swings it towards coax.

Virtually every resource I could find, where they discuss single ended and differential connections agrees, co-ax for single ended, twisted pair for differential. Blue Jeans which appears to get technical direction from a former Belden Engineer agrees. His most recent RCA cable is not a twisted configuration as well. More a modified co-ax.

p.s. bringing up KHz or kHz is pedantic. It is like complaining about spelling / typos in a technical discussion to deflect from the content.

As I clearly said, from the speaker standpoint, the speaker itself is balanced, however, if you are considering noise on the speaker cables induced back into the amplifier, it most definitely is not balanced because the impedance on either cable is not the same as the other. 

Impedance between what and what?.   Induced noise current flows in the loop  output - speaker wire - speaker terminal - speaker impedance - speaker terminal - speaker wire - output impedance - output.  In this loop noise currents will cancel, even if you earth ground it at one point (as long as it is one point only).  It is just simple loop with pickup (induced current) proportional to area between wires. 
How, on earth, noise currents IN THE LOOP can cancel for the speaker only and not for the amplifier.  Loop currents either cancel or not.  If you claim that currents in one leg/wire is different because impedance to ground is lower, then they wouldn't cancel for speakers as well.  How induced noise current in one wire can be different than current in another wire if it is loop with wires exposed evenly to external field, when loop impedance is same for both?   Grounding this loop in two places is different story because there will be additional return path.   Sorry I cannot explain it better.

Perhaps mentioning "KHZ" is pedantic, but it make me suspicious about your experience in electronics.  None of my fellow engineers would write it like that.  As for "deflecting" from technical discussion - it deflects less than bragging about PhD title.
(to be really pedantic:  it is 16k  not 16K)

I absolutely guarantee your fellow engineers will write it as KHz. I just did a quick scan of emails from my engineers and I can find many instances of it. It is probably why I do it. Most of these are MSEE and some PhD. I am working on engineers from 3 continents and who knows how many different backgrounds.

w.r.t speaker wires feeding back into the amplifier, or a bridge or phono catridge, you keep saying loop. If there is a ground connection. it is not a loop. This is the mistake you keep making. It is not a loop. It is not about the induced currents being different. It is about where they go once the meet the single ended piece of equipment, i.e. the amplifier, the phono-amp, the bridge amp (which would never be single ended). At the amplifier the resistance, I guess ideally impedance, must be the same so that the induced voltages are the same, or they do not cancel. If you have a ground connection, or simply a difference in termination, as any single ended piece of equipment will be, then you will have an induced voltage difference from common mode injected currents.

Let’s go back to our speaker connection. I have common mode current that goes towards the speaker. Both see the same impedance at the speaker (it is a floating speaker after all). The voltage on both sides of the speaker, for argument/illustration raises the same amount, hence no change in voltage across the speaker. If the current goes the other way, one side sees the impedance of the amplifier. The other (ground) sees the the impedance of the amplifier, but also sees a parasitic path through through that ground and out to "somewhere". Now you have a differential voltage caused by the common mode noise.

This is common mode. There is still the superior symmetric coaxial structure for rejecting the generation of differential currents from electrostatic fields and even for magnetic fields, the coaxial structure provides rejection.

Let’s approach this from an engineering perspective. My search, though limited, resulted in a consensus that for single ended connections, which we are talking, and which you cannot hand wave away a ground connection that is a resistance/impedance significant with respect to the termination resistance/impedance, that coaxial cables are superior for rejecting external EMI. If I am wrong, and what you say is true, there should be ample evidence of this on the web one would prefer with measurements. Here are some measurements done by an engineer who posts on ASR (not Amir/a mod). I also have another posts from a cable company where they talk about most RCA connections being fully grounded (I know I saw that in the past when playing around with tube equipment). That throws out that big resistor to ground in all cases argument and makes the case strongly for my system dependent argument.

Perhaps mentioning "KHZ" is pedantic, but it make me suspicious about your experience in electronics.  None of my fellow engineers would write it like that.  As for "deflecting" from technical discussion - it deflects less than bragging about PhD title.
(to be really pedantic:  it is 16k  not 16K)

kHz would be it, but I needed to make the K lower case as the spell check caps it.

That is probably the simple explanation of why it got all uppercased.