1. collection of Fastboot and OTA images at one place (use Google translate and click on "official firmware" )
http://4pda.ru/forum/index.php?showtopic=962154. Though there are Fastboot images with more recent versions available, so it doesn't make much sense to extract old OTA images yourself (unless you want to try it).
2. tune2fs must be run as root (adb shell; su; tune2fs)
Well that was almost 48H wild ride
lol looks im too dumb to run adb shell as root cuz after adb shell I always got 'not found' also couldn't find the actual up to date ota on 4pda
So I got slightly older ver. ota gave up on the tune2fs step and headed on extracting the system.img from the payload.bin then flashing it blindly using 'fastboot flash system system.img' and when done got thrown back to yesterday - no phone at all just plain xiaomi logo yayyy fml!
But! on the same 4pda I stumbled upon the latest V10.3.17.0.PFQEUXM Fastboot recovery, and after poking around the web re-flashing the whole damn thing with the stock fastboot rom seemed like not such a bad idea at this point of my frustration.
By now the whole a/b slot tuned into one huge salad in my head so I went for sure and this is what I did:
1. boot to fastboot manually
2. 'fastboot flashing unlock' (holding vol down to return back to fastboot)
3. 'fastboot' flashing unlock_critical (holding vol down to return back to fastboot)
4. 'fastboot --set-active=a'
5. flashed the extracted tar folder with MiFlash and holding vol down near finish to return back to fastboot
6. 'fastboot --set-active=b'
7. flashed the same extracted tar folder again with MiFlash and holding vol down near finish to return back to fastboot
8. 'fastboot flashing lock_critical'
9. 'fastboot reboot'
And hooray! It booted to latest updated rom will have to wait for next OTA update to see if it works at all :cyclops: