@harha
You are ignoring "In general," "Generally," "normally," "Typically," etc.
What you missed is that the IRS can audit a tax return for 3 years after the filing date or the due date, whichever is later, or 2 years after the tax is paid. Since you just made the final tax payment now, they have another 2 years to audit the returns. The amended return does not extend the statute of limitations for an audit, but paying the tax does extend it.
The 3-year statute of limitations applies to amended returns that claim a refund. If the amended return shows a payment due you can file it anytime, and the IRS will accept the payment and process the amended return.
The IRS processing backlog doesn't matter. Your 2018 amended return is considered filed on the date they receive it, even if they don't get around to processing it until several months later. So it is within the 3-year limit. But again, there is no limit on how late you can file an amended return with a payment due, so the 3-year limit doesn't really matter.
