First off, Graendal was the one that asked Delana if she knew Compulsion. This would not make much sense if she herself had taught Delana. The Forsaken are very distrustful (as they should be) of their minions, and they are especially greedy concerning AoL weaves. Perhaps the Forsaken's greatest tool is the idea that knowledge is power, and they use this fact to their advantage constantly throughout the series--they don't even know the knowledge per se, but they constantly keep the BA and Darkfriends in the dark concerning plans, weaves, etc.

The only "big" weave that has been taught by a Forsaken (discounting Taim and Demandred--but we don't really know what's going on there) is Traveling, taught by Mesaana to Alviarin, and that was a pretty big thing when it happened. When Delana answered that yes, she knew Compulsion, RJ would have included a "raised eyebrow" from Graendal and some thought that "they [Delana and Aran'gar] must have been closer than she thought" and that she'd keep an eye further open in the future--something to that effect. Instead, we are all just assumed to be complacent with the fact that a Forsaken taught a not-very-noteworthy BA a pet weave.

I agree with DomA that the Graendal realization was a bit fast-paced. The Forsaken are actually pretty stubborn, despite their massive successes in the AoL--Be'lal was even willing to let Rand put his hands around Callandor because he refused to believe his strength and capability. Graendal is not much different, and she would never really admit she had been "wrong" about Rand--even if it cost her her life.

Secondly, we know that Balefire creates a "tingling" in the air that serves as a warning--if I recall, Rand dove out of the way of a Balefire stream at once point (does anyone have this reference?) because his skin prickled in warning. BS made no mention of this, of course, but I think it could have served as the final "alert" button that put Graendal into full-on survival mode. Sanderson made Graendal seem as if she had been living in a hole the past few months and totally ignoring all of the reports about Rand. Almost every single report of Rand would have included a "darkness" that seemed to be trailing in his wake. Her thought process was just rather illogical, and only occurred because Sanderson knew, and he wanted to infuse Graendal with that knowledge, so he just waved his magic-wand, had her pull an Encyclopedia Brown, and allowed her escape. It was just too unrealistic.