So, I waited to do this quest, but it turns out that if you kill Yaga-Shura first, this quest is no longer available as Aun disappears and there's no option to go to Avernus anymore. Is there anyway to circumvent this?
EDIT: Well, you can set BDEEAunPlot to 4 and MelissanYagaKillJob to 0 and get the option to travel to Avernus in this case. Once back in the pocketplane you need to spawn "BDAUN" to get the subsequent dialog to fire. However, she's not compelling enough a character for this playthrough to really work out why she needs to be recruited before Yaga-Shura is killed. It would be helpful if this is a hard requirement that there should be some dialog from Aun saying to hurry or something.