Nynaeve can't "read" residues, but that doesn't mean she doesn't know they're there. All channelers know what residues are, it's just that to most, they're nothing more than that. It's like being a tracker- anyone can see a rabbit track in the mud, but it takes someone skilled to be able to glean any kind of information from them.

We know this from Elayne's thoughts when Aviendha told her about unweaving weaves. Aes Sedai just release the weaves and let them dissipate, because the alternative is really dangerous. (If no one could see residues, Elayne would be flabbergasted that they even exist- she wouldn't have heard of them)



As for your Greandal theory... that's really, really reaching. Even if the gateway was close enough to the palace to sense saidin (It wasn't, it's not like he opened it at her front door. There was a bit of a walk ahead of the man), and even if Aran'gar was there to sense it (pretty huge coincidence, especially since I believe Graendal has remained pretty secretive about her stronghold), going out to hide in the woods after planting obvious Compulsion in an innocuous herald and waiting around to see what happens, and then somehow remotely dissipating said Compulsion (which is only possible if she managed to unravel the weave without seeing it, from far away), all in an effort to... fake her own death?

What would even be the point? If she suspected Rand had found her, she would either have ran or fought- it doesn't really matter if Rand thinks she's alive.