I don't get the problem. These are mutually exclusive:
Typing in the name of the customer and the code finds the customer
or detects this is a new one
In the first case, would you not want to see the existing details? In the second case, there are no details based on what you typed/selected as a search, so no details. Then you need a way to add the base customer information first, then add the details. A subform should work fine in that scenario (if it is properly designed of course).
If you open the form as data entry, you won't be able to see existing main or detail data.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.