In order for the Coupa Connector to be useful it must handle deletions or refresh all data from transactions until they are fully approved and no longer can be deleted.  In it's current state the data is not reliable.  Some examples

Expense line deleted, Expense report deleted, Requisition line or record deleted, Invoice line or record deleted.

It seems like you could either take the approach you do with Netsuite deletes or maybe easier do a full refresh of all transactions req, expense report, invoice (maybe others) until they reach a fully approved state where Coupa then restricts deletion.