Stingtail is an NPC that is critical in no less than three different quests, complicated by the fact that Sebille will try to kill him. I was playing as Sebille, so that may change the equation a little bit.
The key is the order of quests to tackle. The second key is unchaining the Red Prince and Sebille from the party *and* each other and putting them far from each other. I first used a non-Sebille or Red Prince party member to progress the Imprisoned Elf quest. Then I had Red Prince talk to Stingtail to progress his personal quest. Then I had Sebille begin her murderous interaction to progress her quest.
Since I was playing as Sebille, I had more control over her. If you aren't playing her, you might have to deny her initial request to speak to Stingtail alone, which may lose attitude. If you *are* playing her, keep in mind that you have to beat information out of Stingtail *before* killing him. If you just choose the option to immediately kill him, then you won't be able to progress her quest.
