Well, I can't really fault you for expecting Murphy's Law: that's just good planning
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But again, for the vast majority of MQs I don't think interruptions are even a concern. Certainly in places like Skyshrine, it's unusual to even see another player on the run to the quest NPCs, and if someone actually interrupts your turn-in it's all but guaranteed they're doing it to deliberately disrupt you (and the GMs will be talking to them soon).
Kael is maybe a little unique in that you do have to consider it ... but again, MQs do not take long. Person A gets the trade window up, and says "rdy?" Person B looks around and sees the door to the quest NPC room is closed,so they say "yes".
The time it takes person A to click the turn-in button, then for them to say "done, go", and then for person B to do the turn-in ... I mean even if someone does open the door to come and pull right at that exact moment, they
still probably wouldn't be able to interrupt. Certainly not if you're trying to do this all quickly.
Again, I think what you need to worry about are faction issues and people trying to con you (they kind of amount to the same thing; an idiot who doesn't understand that his faction is too low is functionally identical to a person trying to con you) ... and also the case of someone doing half the turn-ins for that quest previously.
Really there's nothing you can do to address that last one (killing the NPC used to help, but it doesn't anymore), except do the turn-in right after a server comes up. And it's
that possibility that leads people to do the 50/50 thing.