That scenario is much more plausible, I think, than the one that places Graendal (or any other FS) in the manor with SH.

In fact, if it weren't for BS's Q&A comments on the matter, I'd think this was the most likely scenario. It's just that, BS's comments seem to imply that it was SH who gave her the knowledge of the weaves. So, if I were a betting man, I'd put my money on SH (even though it does give him some new funky mind powers). But I wouldn't bet a lot, because I'm far from certain.

The only way Graendal would have known that teaching Elza was in line with her instructions (and therefore that we, the readers, should associate her involvement as such) is if she'd been told what was going to happen after Elza learned the weaves. But, I highly doubt she'd have been told that information - there would be no reason to tell her, and the Shadow is not known for its willingness to share information with those minions who are not in a need-to-know position.

Still, if there was a teacher, Granedal, Cyndane, and Moggy are the most likely candidates. And (presumably) you're right about Graendal being closest in proximity, of the 3.