We know that the Emissaries has requirements or the veiled woman choosing the main character, would have been pointless. He/she is haunted by the past in the dreams, so maybe thats the requirements for prophet? Same for Tealor and dreams of an empire and so on.
As for the high once being powerless, it could actually be true. If the only once that can see them, are the Emissaries, so would there influence not be more then what those people make it to. Assuming the black stones and the red madness, dont let them influence anyone directly. Granted, the Emissaries cant be killed by normal means (if they are already dead), but we have seen means to seal people into objects, so that means there influence is not guaranteed.
The bigger question might be, who the veiled woman is? After all, she effects the main character almost as much as the high once, if not more. My guess is that she is a high one that where created a bit differently (maybe even the first one) and seems intent on nudging events towards the high once being banished. Why else would she do things like taking the main character to the black guardian (one of the few people with the knowledge needed to set things right)?