Alright, my Kindle Scribe Odyssey finally ended!
Thank you Marek very much! I ended up using your upgrade-then-downgrade approach.
Here is the summary:
1) Brand new Kindle Scribe - do not connect to network yet.
2) Connect to a public hotspot that requires login in a browser.
3) Kindle Browser will show up. In the address bar type
;enter_demo then hit enter on the keyboard. Restart Kindle (long-press, window pops up - restart).
4) Follor JB instructions in this thread's OP.
5) After the Chinese language selection, it will put you back to demo.
6) Do not leave demo yet, install both Marek's and NiLuJe's hotfixes first.
7) Then type
;demo then hit yes. It will ask about demo but actually let you in normal mode.
8) Here connect Kindle to PC and drop in NiLuJe's hotfix.
9) At that moment we are in the Amazon Out-of-Box Experience screen that lets you sign-in but doesn't have menus, search bars or other meaningful buttons. So, go to the Browser again as in Steps 2-3, this time type
;711 . A screen with network info will show up, but we only need to hit three dots in the corner - Update Your Kindle. I don't know if this hotfix reinstallation step is necesary, but just in case...
10) Restart Kindle Scribe TWICE. Do not sign-in anywere yet. This will make sure JB survives.
11) Sign-in to Amazon, and agree to install updates - this is the annoying part because of which I spent 26 hours in a row (with some sleep in between)
12) Once it updated, go to Airplane mode (to prevent any other updates). Goal is to NOT let happen another update OR factory restore before JB is recovered.
13) Use NiLuJe's K5 Jailbreak package, only extract emergency-bridge-recovery.zip's contents (four files) to Kindle's storage.
14) Restart Kindle, check that four packages are gone.
;log mrpi commands can be used again.
15) Install NiLuje's Hotfix via mrpi.
16) Use this script and run it with
;log runme :
https://www.mobileread.com/forums/sh...&postcount=469
17) Now that FW downgrade is possible, download earlier FW version. I went with 5.16.2. "Update" Kindle normally via Update Your Kindle (UYK).
18) Then install Install NiLuje's Hotfix via mrpi or UYK.
Good luck!