coherency with stunned model, target wounds

Re: #1 -- stunning only happens when there's a single model in the unit. Per the rules, under "Groups & Wounds":

"When a unit with multiple models takes wounds, each wound kills one model, until only one last model remains. Only the last model then accumulates wound markers and rolls to see if it’s Stunned or Knocked Out."

For #2 -- The rules say "If at least one model in the unit has line of sight to an enemy model, and has a weapon that is within range of that model, then that enemy is a valid target." Thus, the attacker does not target specific models in the unit (unless they have Sniper or similar), they target the unit as a whole. The defending player determines which models are removed as casualties.