In case someone else runs into this in the future, I just got good news in the mail today from Austin TX:
Dear Taxpayer:
Thank you for your amended return.
We have adjusted your account as you requested.
I logged in to my IRS online to look up my 2019 tax records, and sure enough it shows my second amended return filed in Apr 7 2021 (I mailed it Apr 3 2021 with standard postage), with a post-dated 10-01-2021 "refund issued" equal to what I expected (original amount owed plus mistaken payment), plus interest. So am looking forward to seeing this refund direct deposited "in four to six weeks" as the letter said.
Before this day, for a few months I checked the IRS online records and all it showed me was my rejected amended return from Apr 2020 and a negative balance for my mistaken payment back in March this year.
Am glad that this finally ended up getting resolved!
