When you go to retrieve the Trumpet, there's no need to choose sides in the Raiding Party battle. As Darth Sidious said in Star Wars 2, "Wipe them out! All of them!".
There are no negative consequences to killing off both sides in this battle.Also, just for fun, it is possible to do the Elf King's and Queen Catherine's quests in a reverse manner.
After the Elf King tells you to get the Fort plans, go and tell the Queen about it. She will give you a set of false plans to take back to the Elf King.
By the same token, after getting Catherine's Imprisoned Spy quest, you can go tell the Elf King and he will assign you a double-agent disguised as the Queen's spy to take back to her.
I don't remember if you get the same monetary and XP rewards as if you carried out the quests the normal way.
