[Mate 8 Q&A, Help & Troubleshooting] Patneu:
YEEESSS!!! :victory:

Tried flashing EM8_40416_SIGNED.zip now and went through without errors. Took a few seconds to reboot and it works! Just B170 now it seems, but hey...

My quest to upgrade may have failed graciously, but at least I got a working rooted phone again with Kang Tools - what more can I ask for? :D And unless someone, somehow, somewhen has a hint for me about this damned 'image verification error' (at least I now know a foolproof way to fix it if I mess anything up again) I'm taking this cautionary tail to heart and won't touch this damned phone's firmware again 'til it turns to dust under my fingers. ;)

And now, as common courtesy commands, my exact steps - from having completely messed up with nothing save stock recovery and fastboot mode working anymore to functioning phone again with root, cutom ROM and Kang Tools - for the sake of the poor souls of the future who may (hopefully not) follow in my footsteps facing the same problems (no guarantees or responsibility whatsoever on my part, just what worked for me):

1. Reboot your phone into fastboot mode (bootloader) by pressing and holding power + volume down together (don't forget to connect the phone to a PC before!). As always, make sure you have enough battery power left before each step! If you don't at this point, try booting into stock recovery by holding power + volume up and charge your phone, then continue.

2. Download:

2.1: DC Phoenix from here. Found here via here.

2.2: File with partition rework and factory firmware from here. Found here.

2.3: NXT-L29C432B180 stock ROM from here (you may need a different version depending on your device!). Found here. (Stock ROMs B3XX or higher may work too, but didn't for me - see above).

3. Buy 15 credits here to use DC Phoenix (it's worth it!), unpack and start DC Phoenix and login with your DC-unlocker credentials. (Edit: If you want to try the new guide mentioned below from step 5 onward, you should consider buying the 'DC-Phoenix + HCU timed license' option for 19 bucks instead.)

4. Follow this guide. You find the mentioned 'UPDATE.APP' file in the stock ROM ('update.zip') file you downloaded before.

5. You should have unbricked your phone and have a working stock ROM again by now. :fingers-crossed: Now on to getting a custom ROM again:

(Edit: I fiddled around with my phone a bit more in the meantime, so if you prefer flashing a TWRP on a B5XX-based stock ROM and finding your way from there, you may want to follow this new guide instead.)

6. In the settings of your phone enable 'Developer options' -> 'USB debugging' if you haven't already (and 'Enable OEM unlock' too just to be sure, don't know if it's necessary, but I did it). If you don't see 'Developer options', tap on 'About phone' -> 'Build number' a few times. Connect phone to your PC.

7. Downloads:

7.1: SRK Tool from here. Found here via here.

7.2: Ephemeral Mate 8 from here. Found here via here. (Later version or other ROMs may work for you, but didn't for me.)

8. Move 'EM8_40416_SIGNED.zip' file (or the one you want to flash) to the external sdcard of your phone.

9. Unpack and start SRK Tool.

10. Select '3. Install Recovery 6.0' -> '2. Huawei Mate 8' -> '2. Install Mate 8 TWRP for Marshmallow (EMUI 4.0)' (may depend on the stock ROM version you flashed before). Confirm requests and follow the instructions.

11. Your phone now reboots. Remove the USB cable connecting it to the PC and press and hold power + volume up to boot into the newly flashed custom recovery.

12. Select your language (it was chinese later on for me, when I didn't!) and press 'Cancel' if asked for a password. When asked to allow changes to system, do it (don't know if necessary, but I did).

13. Select 'Wipe' -> Swipe to Factory Reset'.

14. Go back and 'Format Data' -> type 'yes' and confirm. Reboot the phone and enter custom recovery like before. Factory Reset again. (Don't know if this step is necessary, but I did it.)

15. Go back and select 'Install' -> select and flash the zip file you moved to sdcard before. If no errors occured, reboot your phone. Let it reboot, it may take a few seconds. If you get errors, try starting with step 1 or 11 again (depending on how badly you messed up...) and try a different custom ROM (Follow the instructions there and be aware of prequisites and warnings!). If that doesn't work either, note your errors and ask for help - good luck!

16. Congratulations, you should have a working phone again with Ephemeral Mate 8 custom ROM with Kang Tools, it should be rooted already. :) :good:

17. Maybe buy the guys, who developed and wrote the mentioned tools and guides to help you on your way, a coffee - they surely deserve it. ;)

Hint: If you only want a rooted stock ROM, reboot your phone after step 12, start SRK Tool again, select option '4. Root' and follow instructions.