Yes. There is no harm in trying to abate the excise tax for the excess Roth contributions by amending closed years, 2014 and 2015 to change your filing status from MFS to MFJ. You can amend all open years, 2016, 2017, and 2018, too.
While it is true that the IRS usually will not accept an amendment after three years, you can request an exception. If denied, you can reach out to a Tax Advocate for assistance.
[Edited 2/29/2020 | 1:51 PM PST]
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