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

I am not ignoring that it is a coaxial cable because it is not a co-axial cable -- the axis of the conductors is not consistent in a twisted pair with shield. Much of the benefit of a coax is the symmetrical arrangement of the shield w.r.t. the center conductor.

A shield around a twisted pair provide electrostatic shielding, but the question was RCA, and the two conductors (as noted by Kijanki) do not see the same electrical impedance which will defeat the benefit of twisting (at least for magnetic I/F) which works where both conductors see the same impedance hence induced voltages cancel. There is probably a reason why oscilloscopes probes, which are single ended, use coaxial cable and not shielded twisted pair.

I suspect that reason for oscilloscope to have plain coax is capacitance. Twisting wires reduce inductance but increases capacitance - important with scope’s high input impedance and very high measured frequencies. Unfortunately this coax arrangement creates errors. When you short probe and touch tested circuit with shorted leads it will show small amount of noise (in spite of being shorted). It is because current flows from the circuit thru the shield (finding return to GND) causing voltage drop, that shows on the input as signal - exactly what we try to avoid in interconnects by using two wires inside of the shield.

Because twisted pairs provide cancellation to all external fields (magnetic or electric) it is used everywhere - in all network cables, in all audio cables etc. It would be stupid not to. Typical twisted pair provide about 40dB rejection up to about 100kHz and still 20dB rejection at 700kHz. Above that shield becomes very effective by means of skin effect. Skin depth changes with frequency squared, being roughly 2mm at 1kHz (Cu or Al). For 100kHz it will be 0.2mm. There is some info on twisted pair effectiveness here:

So what happens if you Connect the ground to the connectors on both sides of the RCA cable? If one side of the ground is not connected to the other side within the cable wouldn't the impact of a ground wire only connected on one side become an antenna?

Isn't it Better to let the ground flow back into the component and let that take care getting rid of the ground activity.

All those examples you gave Kijanki are differential connections where the impedance in each leg is the same. EMI induces a current. The equal impedance means those equal and opposing currents generate equal and opposing voltages which cancel.

The op stated RCA which is not a differential connection. Ground and signal connections have different impedance hence the induced voltages from the equal and opposing currents do not cancel out.

Twisted pairs makes total sense in balanced audio connections. It will be better than non twisted for single endednbut coax is likely to be superior.

In single ended connection we are passing signal and reference point (analog ground). There is a loop from the output to input and back by return - analog ground. Induced electrical noise currents in both wires flow in the same direction and cancel. This would work perfectly if there is no other path for return, like when chassis on one side is not earth grounded (II class) or when it is grounded but analog ground is floating on either side. Analog ground is often connected to chassis ground with the large resistor. That would diminish effectiveness of twisted pair a little but still, twisting would help and perhaps that’s why many manufacturers twist single ended wires inside of the shield.
What balanced connection bring is complete symmetry and independence from this additional return path, but also, when properly done (signals not referenced to ground) removes effect of wires to shield capacitance (allowing for longer connections).

Another example of single ended output, that behaves like balanced, is amplifier output, since speaker is floating.