Final summary:

So, my experience counters some of what I have been told by the fellow Linuxheads.

1. You actually do not need the MiFlash tool to succesfully flash Android back.

2. No matter whether I used the MiFlash tool or manual fastboot commands in Linux terminal/Windows cmd, the result was frustratingly samey: Going back from Linux, Android 9 made the Wi-Fi unaccessible.

3. The good news is, Android 10 -- again, no matter how I flashed it -- didn't bring around the same trouble. Alas, Android 10 was the reason Mi A2 got significantly slower in early 2020, so I still wanted to go back to Pie.

4. If you struggle with the same set of circumstances, there is a hope! I flashed Oreo (which, just like A10, caused not troubles) and updated to Pie from there.

Hopefully this may help someone some day!

Hi,

the title says it all. How do I turn on the USB debugging on UT? What needs to be done and how before I flash Android?

As I suspected, "Flashing is not allowed for Critical Partitions" after my short affair with UT. How to enable flashing Android again?

Seems "fastboot unlock critical" did the job.