The DO can't pass orders mind-to-mind. He's still trapped in SG, and therefore needs an intermediary to receive the orders at SG and then go to the troops to deliver those orders. Even though the DO's influence on the Pattern could be reducing his need to have the trollocs' fear of the Fades keep them in line, he would still need some sort of intermediary to pass his battle orders from him to the troops. He can't pass orders mind to mind... he still needs someone to carry the orders from Shayol Ghul to the armies.

It doesn't have to be a Fade to do it. If there are not enough Fades to go around, as was implied by the lack of Fade-swords, then it can't be always be Fades doing it. It could be BA, dark Ashaman, Forsaken, etc. It would have to be people who can channel, because the trollocs would simply ignore or eat anyone too weak to control them: Despite the DO's growing influence, the trollocs are still as unruly as ever. Alviarin was marked by Shaidar Haran with a True Power sign that forbids any trolloc from attacking her (although it does not compel obedience from them, the way the Forsaken's DO marks do). The fact that she was given this mark indicates that the trollocs are not so reliable after all. They still need external force to keep them in line; True Power markers on DFs to prevent them from attacking, True Power makers on Forsaken to compel obedience from them, the fear of the Fades and the weird bond thing with them, etc. The only person who has ever controlled the trollocs without being able to channel is Fain, who had the power of Shadar Logoth instead.

I dunno what Narg's deal was. I always thought it was an EOTWism. I.e. something introduced in the first book that is forgotten in all subsequent books and never mentioned again (like the affinity a healer is supposed to feel with those they have healed.)

Long story short, the DO still needs someone to pass orders to the trolloc troops, because he can't communicate from SG directly to them. He also still needs someone to keep them in line, because the evidence is that despite the DO's growing influence on the Pattern they are still just as unruly as ever. It doesn't necessarily have to be a Fade to control the trollocs, but it does have to be someone strong enough to control them without getting eaten. That would pretty much require a channeler. If in fact there are not enough Fades, then this would mean that the DF channelers are having to pick up the slack.