It would seem that a heroe in Rearward Stance is not considered engaged.A character is engaged when paired in close combat with at least one opponent. A combatant remains engaged until he defeats all opposition.
I alway play it so that anyone in a Combat Stance (even Rearward) is considered engaged. That way, everything else seems fine. In your example, one of the fighters in a melee stance would choose the third orc his second objective and the wounded character could go into Rearward.
But I agree: As is, there's a need for some clarification.
Otherwise, the use of the disengage action (Athletics vs TN10+Attribute Level) seems fine.