Q : If a contract that contains the FAR E-Verify clause,can be completed in less than 180 days, how do Federal contractors who choose to verify their entire workforce comply with the requirement of verifying their existing nonexempt employees within 180 days?

The contractor’s obligations, including the requirement to use E-Verify, continue until the contract is closed not just for the period of contract performance.  As a general rule, just because the period of contract performance has come to an end does not mean the contract is closed.  Please consult with your contracting officer and see the Disputes Act found at 41 U.S.C. 601-613 regarding contract closure and period of performance.