Can a shop have more than one employee card/upgrade card?
Yes, though a shop cannot have more than one of the same Upgrade Card type, and it cannot have more than one employee card with the same name.
What happens if the monster suffers enough wounds to be defeated before all heroes have had a chance to attack?
The monster only checks the number of wounds after all heroes have had a chance to attack and defend. So each round all heroes will have an opportunity to resolve their attack or defense and earn their shop stars before the monster checks to see if it's defeated.