[Shield Android TV Q&A, Help & Troubleshooting] phnikola:
SOLVED

Writting this as soon as I could cool down.

Situation:

- Shield ATV 16GB 2017. SW Nougat 5.2

- Unlocked bootloader (every boot gives that safety warning)

- System bootloop on Android circles animation (managed to root it previously and removed some critical apps by mistake)

- NO USB debugging

- NO custom recovery (as process for rooting only used TWRP to flash SuperSU, nothing else)

- Entering fastboot possible only by hardware method (controller connected by USB, hold A+B buttons while Shield is plugged off 10sec, then plug Shield in. Two short vibrations of controller and fastboot screen appears.)

However - NO adb in fastboot. (pinging with adb devices gives NO response). Only option that works is entering official recovery which gives options for adb sideload or update from USB. However - no zip file is accepted due signature check (and I tried tons of them)

Tools

1. Windroid

https://xdaforums.com/showthread.php?t=2499926

2. Proper TWRP build

http://nvidiashieldzone.com/download/twrp-3-1-1-1-shield-tv/

3. Custom flashable rom (I used Lineage OS for Shield - lite and without Gapps - to speed up booting into environment that works). Put it onto USB drive

4. Oficial Nvidia recovery image (https://developer.nvidia.com/shield-open-source)

5. Minimal ADB and Fastboot (http://nvidiashieldzone.com/download/minimal-adb-and-fastboot/)

6. Nvidia controller plugged into USB port next to HDMI

7. Male-to-male USB cable (for ADB)

8. USB mouse

Process

1. Start Windroid and choose device (Nvidia, Shield TV)

Windroid will automatically download resources in Data folder (incl. TWRP that did not work for me), so dont block it with firewall.

2. Download proper TWRP from link above, and replace the one in Windroid Data/Recoveries folder using same name (Recovery1.img). You will notice difference in sizes. One that Windroid downloads is 25mb, while the one from link is ~ 16mb.

3. Enter fastboot on ShieldTV using hardware method (above mentioned with A+B buttons)

4. Now quickly In Windroid click button Refresh - and status will be updated to Fastboot

5. Then again in Windroid - click Flash TWRP and accept safety questions.

6. After successful flash - reboot into TWRP (here you need USB mouse - so you can unplug Nvidia controller, and plug mouse)

7. In TWRP - Choose Install - navigate to external USB and select flashable zip (Lineage ROM as above)

8. Once booted - enable USB debugging again. (you could be prompted on the Shield TV to allow your PC as debugging device, if so, say yes)

Getting back to full stock ROM with TWRP and unlocked bootloader

1. Install Minimal ADB and Fastboot you downloaded

2. Extract boot.img, system.img and vendor.img from Nvidia stock recovery image (leave other files out), and place these files into Minimal ADB and Fastboot folder (it is in Program Files x86)

3. While on (in LineageOS) connect Shield TV with PC using male-to-male USB cable

4. Start Minimal ADB from destkop icon and run following commands (commands are only in italic)

adb devices (checks if device is online. Its a must to proceed)

adb reboot bootloader (reboots to fastboot)

fastboot flash boot boot.img

fastboot flash system system.img (will take longer as its a big file)

fastboot flash vendor vendor.img

Once finished - reboot to TWRP, plug in USB mouse (instead of controller) to navigate, and perform Factory Reset.

If you wish to root you can do it via TWRP (which is what I did, flashing SuperSU previosly downloaded to USB)

Thats it - you are back from the dead!