That's also one reason why Moridin has to gather the Shadow's armies, not Demandred. As Nae'blis with access to the TP he will probably have less problems to deal with all the Shadowspawn, even if Graendal indicates that some Shadowspawn won't obey any Chosen no matter what.

However, I think it will be actually SH's role to personally lead the Shadow's armies in the battle against mankind at TG. After all, he is likely a powerful fighter (considering his abilities & stature), we see him in CoT in armor, and this would explain why he muses in ACoS that the tie to SG has to be severed; it restricts him if he is in the south. Finally, it would certainly motivate the Shadow's armies, if the DO's Hand & Voice is basically fighting alongside them, and on the other hand further frighten the human armies.

Maybe SH will even have the power to link himself to all Shadowspawn to coordinate their fightings better (like normal Myrddraal can link themselves to a Trolloc Fist). Such a universal link would also plot-wise be a good way to get rid of basically all Shadowspawn after the DO is defeated and SH dies as well.