"Of course, this would mean the 2021 return would be received before my 2020 amended forms declaring the initial 2020 1099-R distribution has been processed by the IRS. "
Note there is a huge backlog of paper returns and amended returns for 2020 and now 2021.
The simplest solution is to not divide your 2020 distribution in three parts for 2020 when you amend.
Then you will not have any contradictions by submitting your 2021 tax return now.
You won't need 8915-F unless you are taking another disaster distribution in 2021.
If you are not getting any refund(s), it doesn't matter how long IRS takes to approve your tax returns.
