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

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.

You are looking at this more theoretically than practically. Excuse me as my expertise is physics (and managing EEs) not day to day EE and a lot of time around semiconductor processing equipment, some of it quite noisy while trying to do precision measurements. We are not passing a reference point. We are connecting the reference point on two pieces of equipment and making them the same. The impedance on the ground connection is not the same as the signal connection. An equal induced current on the ground creates a small voltage only related to the ground circuit. Induced current on the signal creates a voltage a factor of the impedance . You are making the assumption there is not a relevant secondary path for ground, for instance totally disconnected or a large value resistor. How large is a large value?  How large does that resistor need to be to ground so that it is not there?  Poked around enough in tube amps to know that resistor may not be very large at all. What if there is a capacitor?  10 nanofarads? That is only 16K resistance to ground at 1KHZ.