Your 2016 tax return probably should have been amended prior to the April 18, 2020 expiration of the statute of limitations for the IRS to be able to assess the tax on the under-reported income. Since this statute of limitations has expired, I think the IRS is required to send the payment back to you if you do amend. The IRS had the opportunity to detect the discrepancy and assess the tax because the amount in box 1 of your 2016 Form 5498-SA (taking into account any amount in box 3 of your 2015 Form 5498-SA) was less than what you claimed on Form 8889, but they failed to do so before the expiration of the statute of limitations.
The roughly $86 of interest and 50% compensation for the lost tax-savings potential needs to be included as ordinary income on your 2019 tax return. I would do that as Opus 17 described. I probably would not include the $166.66 on the 2019 tax return, but don't take that as tax advice.
