You can file Form 8606 as a standalone form if needed as long as it doesn't affect anything else on your tax return. Please see Form 8606 for prior years.
Yes, it is a good idea to include an explanation statement to explain the error to avoid the penalty for failing to file Form 8606.
If the IRS has any further questions they will contact you.
Yes, on your 2021 return you will enter the correct higher amount.
No, you will have to mail it.
Edited 4/18/2022 | 9:25am PST]
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