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

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.

Is it better to connect the ground to each side of the RCA cable?

Single ended RCA cable, the ground is connected on both sides, there is no choice. If there is an additional shield, typically is it better to only connect it on one side.