Questions about multiquesting

[red_demonman]

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, not necessary to do this.

Is there a window of time in which to turn in items? If so, how long is that window?

From what I understand, the answer for this server is no. The NPC retains the items (even through being killed surprisingly).

Is there a chance the MQ won't work, even if everything is done properly by the players?

Not really, if player A hands the greaves in and player B hands the gems in unstacked afterwards then there should not be an issue and player B will end up with the quest reward.

Is it customary to pay the person before or after the MQ process?

This is up to some debate/negotiation on your part. It all depends on who takes on the risk. Personally, I think it is on the MQer to take on the risk for the MQ.