My suggestion is that, on the armed courier mission, you should be allowed to retreat if you get accosted by an opposing band demanding that you surrender the parcel that you are carrying. After all, your goal is to deliver the parcel, not to slay anyone. Retreat should, in fact, be the optimal solution for this problem. The risk here should be that you have a member of your team that gets caught in a net, has a wounded leg, or that the terrain you have to escape through is an obstacle and you lose brothers.
As a possible option, the parcel could be carried by a member of your team that you select, and if they die and you retreat, you fail the mission.
In addition to being able to retreat, the opposing band for this mission and also for the scenario when you are carrying the head of a bandit leader, should exist on the map, allowing you to avoid them. As a possible option, they could appear friendly, but then attack you when they are in range