1) is especially true given near-field vs. far-field effects. With regard to RF emission, when a non-emitting conductor is in the near-field of an emitting conductor, the non-emitting conductor has current induced in it and re-radiates as if it was a conductor itself. When the non-emitting conductor is in the far-field of an emitting conductor, the non-emitting conductor will still have current induced in it (this is how antennas are able to receive RF transmissions) but it will not re-radiate to a significant degree.
The exact size of the near-field and far-field depends on a number of factors. At 60 Hz, the wavelength is about 16 feet. Assuming your run of cable is at least 8 feet (1/2 wavelength), then the near field is equal to D^2 / 8, where D is the length of the cable. As you can see, the near field gets longer as the cable gets longer, which is why the potential for re-radiation (and induction of EMF) is so high with longer cables. On the other hand, once the cable is past a reasonably short length, you are basically guaranteed that any other cable in the same trench is going to be in the near field, so in a way, this makes the problem simpler, since moving the cable out of the near field is out of the question.
Basically, shielding the cable becomes your only option. But since we are talking about raw DC power lines and not sensitive, high-frequency/low-voltage data lines, the real answer, in my humble opinion, is to have the equipment on the line be sufficiently protected that the EMF doesn't affect it. It won't take much to filter out the induced current from an adjacent AC line, and I think it's likely that your DC power equipment is already doing this. But that's not my area of expertise, and it's just speculation.
Regarding point 2) yes, the twisted cable gives the ability for the equipment at the end of the cable to cancel out interference and cross-talk, but as far as I know, that requires that circuitry in the equipment at either end perform certain signaling techniques, such as common-mode rejection or differential signaling. I'm not aware of any standard AC equipment that does this. As far as I know, these techniques are exclusive to data transmission.
EDIT TO ADD: BTW, the near field of long cables is very large, but the inverse square law still applies. In short, the RF signal gets weaker proportional to the square of the distance. So at a sufficient distance, the re-radiated power is negligibly weak even though the receiving conductor is technically within the near-field of the transmitting conductor.
EDITED TO ALSO ADD: This conversation is what you get when you ask an RF engineer to talk about electrical wiring. There is some overlap there, but ultimately, if an electrician disagrees with me, you should do what he says. But if an electrician disagrees with me as to how to wire up a radio antenna, then listen to me.
