This may require one more call to the employer. If they are not correcting the 2018 Form W-2, then the IRS still has the information from Copy A, and will be expecting to see that income reported on your 2018 Form 1040 tax return.
I think the solution that will have the fewest obstacles in the road will be for you to consider the February 2019 check as a good-faith reissue of a check that never reached you in 2018, and file an amended 2018 tax return to include the Form W-2.
