In case #2 your disclaimer doesn't work. That is, if an army is merely defeated and not entirely wiped out, there's still someone to retreat from the battle in an "organized" way. They may have lost, but that doesn't mean the makeshift area was in their front lines. In the times of this kind of warfare, camps were not really on the battlefield and would still contain people capable of handling prisoners.
Continuing that point, technically only heroes fight with their units. The non-hero nobles would likely lead their units in, give the order, and hang out where their unit was (or in the case of archers, still are). Injuries would happen with arrows (which conveniently can be aimed most places on the battlefield) or when the unit is overwhelmed and the enemy is able to close on that command position faster than the noble can retreat. (I don't think it's possible for a noble to get injured after their unit has somehow been removed from the field.)
Don't get me wrong, I like the "wiped out" portion of this idea, but not the mere "defeated".
