A) Is there a FAR/DFAR definition for Source?
To answer your question, I too could not find a direct definition to "source" in the FAR. As to your background I offer the following.
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Let me begin by saying always refer to the FAR and DFARS when looking for your answers and I applaud you for your search. In your case the FAR that talks to acceptance is FAR 46.503 Place of Acceptance and it states:
“Each contract shall specify the place of acceptance. . .”
FAR 46.401 states
“(b) Each contract shall designate the place or places where the Government reserves the right to perform quality assurance.
(c) If the contract provides for performance of Government quality assurance at source, the place or places of performance may not be changed without the authorization of the contracting officer.”
If the contract identified the contractor in Block 9 of the contract with their address that is usually where “Source” or “Origin” are. If the contractor intends to use a location other than what is in Block 9 they must identify that in writing to the contracting officer and a mod to the contract must be written. There are some exceptions to this rule and I suggest you discuss this further with your leadership and your ACO.
You are also correct that the Place of Performance clause should have been in the contract if the contractor intended to use a location other they what they identified in the contract.
By the way, usually Block 7 is reserved for DCMA, Block 9 is the contractor’s information.