To reply to your questions:
[1] It is unclear at this time whether they will automatically adjust the stimulus for returns that have reported it incorrectly. Therefore, yes, it would appear Form1040-X would be required for stimulus payment adjustments.
[2] No; The first and second stimulus payment reconciliation is done on the 2020 Tax return due April 15, 2021.
[3] Even though Form 3911 is submitted in 2021, it will not impact 2021 because Federal Income Tax Refunds are not taxable. State income tax refunds may be taxable depending on your circumstances.
The second stimulus remains tied to 2020 because the legislation was passed in 2020 as @xmasbaby0 stated. Tax law sometimes changes and is applied retroactively depending on what is written for the law. You could also think of it as an exception made for the second stimulus payment to be received in 2021 but apply to 2020 so that people needing assistance do no have to wait even longer to get the funds that can help them now.
These payments are not taxable. However, some states do count them toward household resources in calculating things like property tax credits.
@Kurt Merkling
