Morning (in Brazil) news: I am beyond pleased and proud to tell you all that the issue has finally been solved!
After our last round of replies, I decided to do a deep dive into all I could find about APFS, the reasons behind this stubborn error message and the most likely solutions. I will summarize what I found out and what was the definitive solution that freed me and my MacBook from the chains of that annoying error.
Previously on this thread: Apple Diagnostics shows no HW error, Disk Utility showed error code -69874 until I formatted the partition in standalone (aka recovery) mode. It then became error code -69842 which seemed easier to fix (could not unmount disk) but proved to be as tough as the first one.
It took me about a week to find the time and online resources to gather a whole bunch of information (it is summarized in a pdf document that I am not sure I can upload here). I learned that Disk Utility is a great tool but is somewhat limited and that Terminal commands are way more powerful (insidious, one might say) than what you can do in Disk Utility.
First thing I did was to prove my theory that there's a partition map that resists formatting, it is simply updated and not wiped/recreated when one just erases a disk with no other changes. So I went into recovery mode, booting from the external HDD and decided to shrink the only partition and force the creation of a second, smaller one that could allow me to delete the offending, original partition.
Unfortunately, that did not solve the problem as I still could not delete the original partition by clicking the "-" (minus) sign in Disk Utility.
I went back to my notes and started Terminal; it was time to either eliminate the error or die trying. Even if it meant actually wiping everything. Of course, at this moment, I had to have (and had) faith in my 3 (up one from last time) external backups.
In Terminal, I did:
diskutil list
diskutil eraseDisk APFS "Macintosh HD" /dev/disk0
diskutil apfs deleteContainer diskXsY
And the message (-69842) was still there! Stubborn little devil, isn't it?
The next option was what I had been looking for all along:
gpt destroy /dev/disk0
diskutil eraseDisk APFS "Macintosh HD" /dev/disk0
The result was more than satisfying!
However, I didn't want to stop there and went on to the next step: ERASE MAC. Side note: this would have done the same thing the previous commands did plus a lot of extra wiping.
From the menu: Recovery Assistant → Erase Mac
It took sometime as it went online to, I guess, download MacOS Sequoia. Finished without issue and rebooted into the Transfer Data sequence.
Took much less time than his time (about 3 hours) and loaded everything back.
The only visible consequence is that the message has disappeared which makes me believe that, at least for now, the error is gone and there's no indication I was at risk of losing everything. Final diagnose: APFS corruption either in Object Map or fsroot (the B-Tree from where all the structures/branches stem).
Thank you very much for all the help you provided and I do hope this thread will someday help somebody else.
-Fernando
