<<It feels like you're saying my 20/20 return and the AGI they're in is a sole determination of how much he should get for each of the three stimulus payments if any.>>
that is close... for the 1st two, the 2020 tax retrun is the sole determinant, for the 3rd payment the 2021 will be the sole determinant.
This may help distill the confusion that I wrote tongue in cheek to explain what is occuring:
the conversation in March, 2020 between Congress and the IRS went something like this:
Congress: Send out $1200 to every taxpayer and base the calculation on the 2020 tax return
IRS:: but 2020 is not over yet, we don’t know what people will submit on their 2020 tax returns
Congress: then use their 2019 tax return (and if that is not available use 2018) to determine the stimulus payments
IRS: what happens if we send out this money and when we analyze the 2020 tax return we realize we sent too much?
Congress: not to worry we will write the law so that if anyone gets more money than their 2020 tax return says they are entitled to, they won’t have to return it! Line 30 cannot be negative!
IRS: Brilliant and Yes Sir, we will send out the checks.
Then in December, 2020
Congress: do it again, same rules but use $600
Then we get to the 3rd Stimulus
Congress: we are going to do this again! But this time we are changing the rules! Each taxpayer receives $1400 for himself and $1400 for each dependent; base it on the 2021 tax return!
IRS: But no one has filed their 2021 tax return, what do we do?
Congress: Estimate what they are due based on their 2020 tax return and if that is not available use 2019. And we will do the same thing we did before, if the IRS sends out more money than the 2021 tax return indicates they are entitled to, no one has to return the extra money!!!
IRS: Brilliant again and Yes, sir!
does that help?
