there is really no traits that requires unit to be put in first position unless stated so in the trait
example: Leader (Gain 3 to all Skills if in first position.)
there are a couple of traits that are connected to skulll damage - and since only the ‘first from top’ unit deals with skull damage then only that troop’s trait is used in those cases but it is not necesarely the literal first position, if your 1st troop dies then its the whatever unit is on top.
example: Armor Piercing (50% chance to ignore Armor with Skull damage.) - only triggers if the troop that has it deals the skull damage
example: Burning (Inflict Burning when enemies deal Skull damage to me.) - only works if skull damage is dealt to that unit
example: Bloodsucking (Deal double Skull damage to wounded enemies.) - only works if the unit that deals skull damage has this trait
i am pretty sure that if you think about it for a while you could see mechanism here
edit: thats how it works on pc/mobile but i assume xbox got the update that suppose to adjust to pc/mobile in this case?