The retreat mechanic allows to retreat only the people on the very edge, unfortunately even being few tiles away is a 50/50 chance. Not to mention that if you join an npc battle and retreat as soon as you join (starting in a corner far from any npc) you still lose half of the men. Not saying it is bad but it is definitely not intuitive and maybe should have some marked zone telling the player where the safe retreat is. Even if the zone only appears when the retreat button was pressed and the text about the safe retreat being on the edge appears.
About the heavy armors helms and such, please do remember this is a light fantasy game and not everything from real life benefits the gameplay. The fatugue reduction made by the armors and helms is significant and you sure need a high level warrior or a medium one with good traits to use them. You do not need to have heavy armor, it is purely by choise. You can make light armored fast moving sellswords like in the “one man challenge” picture. They are valid and useful, it only depends on your playstyle. Rushing heavy armor is not the best option though, you should keep a balance of fatigue to be able to fight at all. There are a lot of perks that help you with heavy armor in utility tree tier2. And still it is possible with right traits and perks to make a heavy armored relative high fatigue member.