Do we have to have all our quests completed simultaneously? I also think trades are an extremely valuable part of quest completion. Using diplomacy to make quests easier is in the spirit of the game I feel. Niv can make the final decision is suppose but to me, trading to gain an advantage has always been a part of the game and shouldn't be removed now.
Example: you get the quest, "control the least number of outposts." Naturally to do this you would want to decrease your initial expansion by at least a base or 2. This will slow your momentum for a long time if not the whole game. However, if you can trade away those outposts for some drillers and not be affected as much. It can also apply with the quest, "have six stacked hires," because you could trade drillers or bases for specs so you aren't limited with your specs.
It also helps some quest complement each other such as "control the most/least outposts" complementing each other. The person needing most would be happy to trade away some drillers for precious outposts, and the person needing the least would be more than happy to receive drillers in exchange for his outposts. Obviously don't tell the person you're trading with your quests but to stop trading during this game seems to me like it'd be restrictive for such a creative game.