Questions about multiquesting

[Soothsayer]

I'm about to invest 10k into a Thurg plate greave MQ, but I've never actually done a MQ before. 10k is quite a bit of money for me, so I'm a bit timid about losing it all and bankrupting myself for nothing. How exactly does the process work? I don't want to somehow screw up and lose 10k for nothing.

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?

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

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

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