There are basically two views concerning, which certainly both have their problems, and everyone has to decide what seems more likely to him.

You apparently believe that there is more going on, namely that Graendal was in the house (either by coincidence or by SH's orders), Delved Elza but didn't remove the Compulsion for some reason, taught Elza the important weaves to open the box, but didn't fetch the DM herself, and left the house again. It seems that there is nothing to support your view, except for your opinion that only a channeler could give Elza the knowledge about the weaves.

On the other hand, I believe that there really was only SH and Elza, as it seems to be in the book. Also all of BS's quotes about this scene only mention SH and Elza, e.g. that Elza was essential for SH, which doesn't really fit with the idea that Graendal was there as well. We know that SH and the DO can give DF gifts and change them. It doesn't seem that far-fetched to me that he could also give certain knowledge, but couldn't remove the Compulsion, which is mentioned as being quite difficult, especially because we don't know the amount of SH's powers.

However, we can't be sure, though I would say it's your job to provide evidence that Graendal was there, since this theory differs from what we see in the book.

Would be a nice question for BS, if there was another Forsaken involved (and as others said, Cyndane, Moghedien and even Mesaana are basically as likely as Graendal then). If he answers "RAFO", there might be something to your idea.