an extended 2016 return normally had to be amended by 10/15/2020. however, there is an exception
Taxpayers may now use a five-year carryback for net operating losses (NOLs) arising in tax years beginning in 2018, 2019, and 2020, compliments of the Coronavirus Aid, Relief, and Economic Security (CARES) Act (P.L. 116-136).
so an unextended 2017 return amendment not involving an NOL has to be amended by 4/15/2021 extended by 10/15/2021
for 2018 change the year ends to 2022 and for 2019 to 2023
however, once an original filing for a tax year is fully processed, an amended return should be filed as soon as possible, less something occur and you lose out on a refund. if you owe it doesn't really matter. the IRS can come after you forever. In practicality, since they usually don't keep returns for more than 7 years, if you are not notified that you owe within that timeframe you're probably safe.
as to entering net wages rather than gross, what did you enter for payroll taxes? your total payments or only the employer's portion. if you enter your total payments which would have included federal withholding and the employer's and employees' share of fica and medicare taxes, then you in total deducted the correct amount - wages understated by $X but payroll taxes overstated by the same amount.
