[Fire HD 8 and HD 10 Android Development] retyre:
Disclaimer: To go through with this, you will have to open up your device to get access to the back of the PCB. This is not for everyone. If you encounter issues, I (or the others here) will try to help you, but the risk is all yours.

First, credit where credit is due. To xyz` for coming up with this and taking the time to help and to k4y0z for helping me get unstuck multiple times.

What's the purpose of this thread, you ask? It's to recover from a bricked 2017 Fire HD 10 as a result of sideloading to a lower version (from anti-rollback). This thread is not about rooting or other apps. Numerous threads of that type exist on these forums. If your device can get to the "amazon" screen or the "Fire" screen, do not waste your time here. Questions unrelated to anti-rollback unbricking will be mostly ignored.

This has only been tested on Linux (Ubuntu 16.04). In general, getting familiar with Linux (as opposed to Windows) can make all the difference in projects like these.

1. Make sure your device is powered off and disconnected from your PC.

2. Take off the back cover, remove the pieces of tape from the battery and display connectors, disconnect the battery and display cables, unscrew the PCB (11 screws), and gently lift it up. Take care not to rip the speaker wires from the board. (To unbrick, you do not have to connect the battery.)

3. Download and extract the contents of unbrick_suez.zip (attached).

4. Navigate to the root of the extracted archive and open a terminal there.

5. Optional: If you see serial port errors, disable or remove modem manager as root (command may vary with distro; try one or more of these commands in Ubuntu 16.04):

sudo apt-get remove modemmanager
sudo apt-get remove --auto-remove modemmanager
sudo systemctl stop ModemManager.service

6. Run the unbricking script as root:

sudo ./bootrom.sh

You should see it waiting for the bootrom. Let it be and do the following with the PCB.

7. Connect the microUSB end of the cable to the PCB. This is the more physically-challenging end of the connection. Leave open the PC side.

8. Short the point (highlighted in blue in the attached picture) to ground. Work with what you're comfortable with, but here's my approach (use M/M jumper wire if you have access to it):

a. Gently nudge one end of the wire into the metal case of the SD slot so that it stays in place (keyword: gently). This frees up one hand. You need just enough grip to ensure it doesn't fall off unexpectedly.

b. Hold the other tip of the jumper wire to the point highlighted in blue.

c. Connect the other end of the USB cable to your Linux box (remove the jumper wire when you're instructed to and press Enter on your keyboard).

You should see the following:

k@Cray2:~/Desktop/unbrick_suez$ sudo ./bootrom.sh
[2019-02-03 12:28:08.466131] Waiting for bootrom
[2019-02-03 12:35:22.602290] Found port = /dev/ttyACM0
[2019-02-03 12:35:22.602653] Handshake
[2019-02-03 12:35:22.603225] Disable watchdog

* * * Remove the short and press Enter * * *

[2019-02-03 12:35:27.691503] Init crypto engine
[2019-02-03 12:35:27.709450] Disable caches
[2019-02-03 12:35:27.709854] Disable bootrom range checks
[2019-02-03 12:35:27.721298] Load payload from ../brom-payload/build/payload.bin = 0x45D0 bytes
[2019-02-03 12:35:27.724457] Send payload
[2019-02-03 12:35:28.262081] Let's rock
[2019-02-03 12:35:28.262834] Wait for the payload to come online...
[2019-02-03 12:35:31.824056] all good
[2019-02-03 12:35:31.824533] Check GPT
[2019-02-03 12:35:33.103565] gpt_parsed = {'lk': (20480, 2048), 'recovery': (174080, 34816), 'MISC': (123904, 1024), 'cache': (3515392, 868352), 'tee1': (22528, 10240), 'dkb': (18432, 2048), '': (0, 1), 'userdata': (4383744, 56687583), 'system': (208896, 3306496), 'PMT': (7168, 9216), 'tee2': (32768, 10240), 'proinfo': (1024, 6144), 'reserved': (124928, 16384), 'metadata': (43008, 80896), 'boot': (141312, 32768), 'kb': (16384, 2048)}
[2019-02-03 12:35:33.103747] Check boot0
[2019-02-03 12:35:34.291300] Check rpmb
[2019-02-03 12:35:34.499043] Downgrade rpmb
[2019-02-03 12:35:34.501403] Recheck rpmb
[2019-02-03 12:35:35.392720] rpmb downgrade ok

It should complete in a few seconds.

9. Unplug the USB cable after "rpmb downgrade ok" appears in the terminal.

10. Put your device back together (PCB and display/battery cables). Do not screw the PCB in or snap the back cover until you confirm your device has been unbricked.

11. Depress the power button (with your nail or a suitable tool) to turn on your device. (If it doesn't turn back on, hold it down for a few seconds. If you hear a ding, that's usually a good sign.) This can be challenging for the uninitiated, but don't complain. Obviously, it's better to verify unbricking now than after you put everything back together.

12. If the device turns back on, you can shut it down and put everything back together. If it does not turn back on, connect it to your PC and see what shows up with lsusb. Time to troubleshoot.

If you have questions, read the two linked threads above. If you cannot find the answer to your question(s), post here. If you append this entire OP to your post (instead of snipping most/all of it), I will, on general principle, ignore your post.