sure you can do the quest again. i have done it 5 times ;-P but i never did try to "complete" it again. i only did the hungry halfling on Keyera two times (once was pre patch so it didn't count), but have done the others..err wait, not trick-or-treat on key again as well. BUT i have done bone collection, bloody vamp and the MM with her many times helping others, and i was almost everytime the quest starter since the rest of my group was bots (helping friends out). so it will give you the quests again. Key has 12 bristlebane cards i belive ;-P
however, the quest ends the 14th of this month........