You're underselling. "Promising" was in Bayou, since then, Shin has taken out high class guys, costing Renpa/GHM their victory - at least the first outcome should be known to Riboku.
If those three are supposed to be the south force, why have them appear here? SHK could have told them of the plan in secrecy and then act like he stations them on the Chu/Wei border. Not perfect, but as you say, once the western attack takes place, Riboku will have his hands full. Them traveling through all of Wei would still be unthinkable.
Now they are part of the attack, if they go "missing", Riboku will have to start wondering whether they try to encircle him/go for another spot.
Also, those three (four with Kyoukai) need a huge upgrade in troops, 15.000-20.000 to travel a long distance, take out elites, take the city and then HOLD this city seems like a stretch. Especially since it is their only foothold.
Or it's just a fake and SHK personally goes to Gyou