Give up on the amended return getting you a stimulus check. You can reconcile it when you file your 2020 return.
The stimulus check is an advance on a credit you can receive on your 2020 tax return. If something went wrong or you did not get the stimulus check this year, you can get it when you file your 2020 return in early 2021—if you are eligible.It will end up on line 30 of your 2020 Form 1040.
And the second Stimulus.....I expect total Chaos....the timing of the second payments, vs including them in the 2020 tax return, it may significantly delay any tax filings in January....the IRS has to figure out how to include both payments on the forms...and the fact you might claim you didn't get the second one, at the same time as it is being sent out to you.......a giant coordination nightmare for the IRS and taxpayers, unless they halt all filing until, perhaps mid-late February.
(or maybe, if Congress was smart, had the IRS delay the final accounting for the second one until 2021 taxes in early 2022.....but I doubt they were that cognizant of the problems they are causing....hope I'm wrong. )