Aren't you in luck. The Reserve Missions are not a part of the 300 you need to complete them all. There are four total Reserve Missions: One to get the Eldagusto, one to get the Vermillion, one to get the Gold Vessel, and My Mission. Usually, each game gets two, on to pick up an item and My Mission. My Mission always requires the two items that you can't get without trading. So you can't complete My Mission without trading. But it isn't one of the 300, you're missing another one somewhere along there.