Editor reads your claim records before each mission, looking for any enemy ace and if found any, it cancels his assignment in the edited/saved mission.
So this is pilot specific? I usually have about 6 pilot careers going at any one time. Pilot 1 may have already shot down MvR, while Pilot 2 may not have. The editor knows who's flying and which Claims Records to check?
Yes, when you edit mission of Pilot 1, editor checks claims record of pilot 1. When you edit mission of pilot 7, editor checks kills of pilot 7. Editor knows what pilot is flying - you can simply test it: click Edit pilots button and Editor will display current pilot's squadron roster. If you click on roster pilot/observer name, Editor will show his personal data: