So I assumed POS to Google Wallet communication was done, but Google Wallet seems to need more information.
Correct. Google Wallet retrieves information about transactions trough the card issuer's online backend system (at least that was the case with pre 2.0 version of GW, but I would assume that's still the case). Thus if you do not perform a real transaction (one that runs through the Master Card network), Google Wallet has no means to retrieve details about the transaction for the online backend.