Hi, I am going to address your "issues" one by one:
Issue (1): Yes, if you start over completely, your 2019 will again be imported exactly as it was the first time around. Part 2 of your issue #1, you state that you would like to "move" these 2 2020 files - is is safe to assume that you are working off of a desktop version? If you are, definitely rename them and save them to your C Drive. If you are not, and are working strictly online, then the only thing you'd be able to save are the PDF versions of what you've prepared thus far. But, I think by how you worded your questions that you are working off of a Desktop version. Please correct me if I am mistaken
Issue (2): You are literally starting over again with a "clean slate", so to speak.
Issue (3): Now, as for the previous years, if you calculate what you should have gotten back verus what you did get back, and it is worth your time - then DO IT!!! Definitely worth it! 1040x are actually rather simple and quick to do.
IRS 1040x Instructions pages 4 and 5, specifically, and
"Can I file an amended Form 1040-X electronically? You can now submit the Form 1040-X, Amended U.S. Individual Income Tax Return electronically using available tax software products. Only tax year 2019 Forms 1040 and 1040-SR returns can be amended electronically."
1040x PDF So you can see what the form looks like and how "simplified" it is. I have every faith in you!