Graendal was either in Caemlyn already, meeting with Rahvin about their plan to capture Rand. In fact considering Moggy knew about the attack, she may have been there to help Rahvin. Then when Rand came into the throne room she hid. After the battle she stuck around to see what she could learn. Or she came in after the battle to snoop around, just as she does after Sammael is killed. In that instance she also has to worry about running into Asha'man on top of the AS and Rand himself, yet she's still there and no one ever questions that. It's the exact same case here, she can't trust BA to not keep angreal for themselves.

As for killing Asmo, he was unlucky to find her hanging out in a back hallway. Odds are she was sticking to the less traveled halls to avoid Rand, but Asmo in his hunt for wine took the shortest route he could find. As for why she didn't use Compulsion, in LoC Sammael mentions that while holding the Source Graendal's tricks couldn't affect him. Also, by immediately resorting to balefire she doesn't have to worry about Asmo defending himself. Balefire also explains his reaction, he senses it coming, just as Be'lal does back in tDR.