Since the employee LET ME DRIVE AWAY without a complete order, how do I go about getting my money back, and an apology from the young individual who took my order and ASSURED me there would be no allergy issues since at least one item was safe?

You should tell the manager that he is the worst

Either call the store and see what solution that they will offer, or go back there and walk in and talk with the manager. You can always call the customer service number and email them at corporate McDonalds as well.

You could try complaining on Google.