In the context of testing a major enhancement to an air traffic control user interface with a limited window for regression testing, the best and most cost-efficient approach to mitigate the risk of the test suite not completing overnight is to analyze the regression test suite and remove test coverage duplication and redundancy (Option C). This approach streamlines the test suite by eliminating unnecessary tests, thereby reducing the overall execution time without compromising the test coverage's integrity. It ensures that each test case provides unique value and that the suite as a whole is optimized for efficiency. This method does not require additional resources or environments and leverages the existing test suite and infrastructure, making it a cost-efficient solution to address the potential delay in starting performance and security testing due to extended regression test durations.