1. Unit of 3 models suffers a hit resulting in a stunned model - are remaining models in the unit now unable to move away until the stunned model recovers (except for minor move within 2"/6" coherency) and therefore is able to move with the unit? Essentially a stunned model in multi-model unit will anchor down that unit for one full turn.
2. Unit of 5 marines is shooting at group of 5 orcs. 4 marines have LOS to 2 orcs and score 3 hits. Do orcs take all 3 hits considering that only 2 were exposed to shooting marines? Who determines which models to remove?
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.