Does an online retailer have to honor their advertised price if the item is in stock my account was debited and the items were shipped but reverse shipped back before I received them?

Yes they do. Once your payment has been accepted by the retailer, they have a binding contract with you either to ship the goods you ordered, or refund your money (or issue a credit note). Contact the company and ask them for a detailed explanation as to why you've not received what you ordered. If they simply refuse to (a) offer an acceptable explanation, (b) send the goods or (c) refund your money - contact a consumer advice organisation to act on your behalf in preparing a compensation claim against them.