I'm in the exact same situation where I used the non-filers tool thinking the "cannot be claimed as a dependent" meant for 2020 ( since that's when we need the stimulous $, not last year; then I got a check for $1200 but my parant cant efile bc, as you said, the non-filers tool actually submitted a full blown 2019 tax report for you, in which unbenownst to you, you're claiming one exemption for yourself; thus your mom cannot claim you as her dependent unless she mails her 2019 tax return through the US postal service and explains the non-filers tool failed to warn you in so many words you would be claiming yourself as one excemption (dependant). So, regular mail for her, yes, e-file, no, b/c you already took up your own excemption.
My case is exactly as yours. My dad will submit his return through the US POSTAL SERVICE claiming me as his dependant for 2019. He had to request a time extension for filing as the system rejected his return through e-file on July 14th. he will explain the confusion in using the non-filers tool (for the stimulous $) in writing and I plan to submit an amended 2019 return for myself requesting zero excemtions for my 2019 return.
As for the check I received, the IRS will have to guide me where and when to send it back. through researching, people in our situation ( 2019 dependent, 2020 independent working) should wait till they do their 2020 return early next year to get their stimulous $ ( the one you and I just got a check for in error). As you correctly put it, our confusion is preventing our parents from e-filing their returns with us as their dependants, thus the get around of regular mail with the appropriate explanation to the IRS.
the IRS form to amend our returns is: 1040-X
