Lets assume that Graendal had Aran'gar with her.

Idiot emissary comes. Graendal knows he's been sent by Rand just now because Aran'gar could sense the Gateway.

She compels the emissary, but not very strongly, (Nynaeve says:
She hissed and said, "Yes. Rand,
he's under a heavy Compulsion. There are a lot of weaves here. Not as bad as the chandler's apprentice, or maybe just more subtle." .) It is likely that Graendal used some very complex compulsion that would not leave the mind fo the victim useless. And, knowing that there may be some trick, she disguises herself and follows him with Aran'gar.

When Rand arrives, Graendal and Aran'gar are not far away, though they're at enough of a distance not to be seen by Rand (and they've cloaked their abilities).

When Rand channels through the CK, Aran'gar knows. She also knows what he's weaving, since there's a moment before Rand sends the Balefire against the palace.

Granedal does the quick thinking she's famous for and realizes the purpose of the emissary.

Now, here's where things get hairy. We know of no way for a person to un-knot their weaves from a distance. But are we sure that's impossible? Could it not be that from her hidden location, Granedal removed the compulsion. Since it wasn't too strong, Ramshalan's mind was not screwed, and Nynaeve was able to report that it was removed.

Notice that some time passed between Rand's balefiring of the palace and Nynaeve's testing of Ramshalan. That time is sufficient to account for the removal of the compulsion.

This is what I think happened. And Graendal reported to Moridin, who must have known Rand is now ready to use the CK any which way.

In the next book, we'll find out what he planned to do with the info. Only, Rand is no longer that crazy.