Steve:

I'm having a hard time following your logic. It starts out okay but then gets lost in "I am fine with the "Bill to" and "Ship to" contact and email fields being identical" as that didn't seem to be what you started out with.

In terms of my experience the Ship-to contact phone and email are rarely the same as the Bill-to so what I would like to see is:

Without a Ship-to code (default or otherwise)

Without Invoice Contact then Bill-to Address and Contact comes from the Customer Master details (I believe that you referred to it has the customer header) and the Ship-to is the same address but the contact fields are empty.

With Invoice Contact then Bill-to Address comes from the Customer Master details, Bill-to Contact comes from the Invoice Contact, and the Ship-to is the same address but the contact fields are empty.

With a ship-to code

The bill-to works the same way but all ship-to fields are determined by the ship-to code.