[Ashintar]

Do both players have to have the NPC's trade window open simultaneously in order for the NPC to "track" what's given to him?
-- no you don't have to have the window open at the same time. The person with the armor piece does his turn in, then tells you when you can turn in your three gems. don't stack them, turn them in one per trade slot with the NPC after the other person tells you he's done his part.

Is there a window of time in which to turn in items? If so, how long is that window?
-- Not sure if there is or not, however I've crashed right before I turned in my gems, came back about 5 minutes later and turned in my gems to complete the MQ.

Is there a chance the MQ won't work, even if everything is done properly by the players?
-- If done properly I don't see how it would fail, but the GMs don't help any more with MQs so I guess there could be a chance of failure...

Is it customary to pay the person before or after the MQ process?
-- Usually people request the money up front.

Just my 2 cents, good luck...