[Nexus Player Q&A, Help & Troubleshooting] snowzach:
So I got sick of Oreo constantly crapping out, being buggy and decided to give 7.1.2 a whirl again just because I remember it working so much better and it's been announced there are no more updates. After fighting with it for a while, I finally got it downgraded and staying there. I found another post detailing some of this but wanted to write a howto from scratch.

Here's how I did it:

Download the factory image for fugu 7.1.2 from here: https://developers.google.com/android/images

Download the latest version of Magisk from here: https://github.com/topjohnwu/Magisk/releases

Download ADB/Fastboot of your choice and install it. I use this on windows: https://xdaforums.com/showthread.php?t=2317790

Download TWRP for fugu here: https://dl.twrp.me/fugu/twrp-3.2.2-0-fugu.img.html

Reboot your nexus into bootloader mode (hold button on bottom while plugging in, light turns on, then blinks, release button) Connect it to your computer with a MicroUSB cable.

First thing you need to do is ensure your player is unlocked. When in the fastboot screen it should show the lock state. If it's locked, you need to run this command twice from your computer: "fastboot oem unlock"

Okay, now you're ready to go, you need to extract the factory firmware and use adb to flash it. Extract the main firmware file. Inside is another large zip file with the broken out recovery and system images. Extract that file also.

First flash the 7.1.2 bootloader: fastboot flash bootloader bootloader-fugu-fugu-02.13.img

Reboot into the flashed bootloader: fastboot reboot-bootloader (If it doesn't reboot in the bootloader, unplug and use the hold button and plug in to get back to the bootloader screen)

There is usually another ZIP in the factory images archivce, you can flash all with the included script or I usually just unzip that archive and flash one at a time.

Flash System: fastboot flash system system.img

Flash Boot: fastboot flash boot boot.img

You can skip flashing the recovery because you are going to flash TWRP with: fastboot flash recovery twrp-3.2.2-0-fugu.img

Okay, now you need to go into TWRP. You can press the button once on the player and it will say recovery mode, then hold the button for a few seconds and let go and it should boot int TWRP.

You need a mouse to control TWRP unfortunately and I had to use a USB OTG adapter to get a mouse connected to the NexusPlayer MicroUSB port.

TWRP Might ask if you want to keep system Read Only. Push the button that says Keep Read Only.

Once you are in TWRP, you need to sideload the Magisk ZIP file. Go to Advanced, select USB sideload and swipe to start it.

Now you need to hook the player up to the computer again with MicroUSB.

On your computer do: adb sideload Magisk-v17.3.zip (Or whatever version of Magisk you happened to download)

It should load Magisk and patch everything for you. I have found that I need to then hook the mouse back up, go back to the main menu, and perform a wipe/factory reset of the Data partition before it will allow it to boot for some reason.

Once that is done, boot up the Nexus Player. It will ask you to connect Wifi and register. If you have non 5g Wifi, connect to that first as you are racing against the clock to install Magisk Manager before the Oreo Update downloads and installs.

The first thing you need to do is allow Unknown Sources from Settings -> Security & Restrictions -> Enable Unknown Sources

Next install Magisk Manager -> Go to Settings -> Apps -> Magisk and hit Open, it should download the Magisk Manager application and ask you to install.

Once that is done, immediately go to network settings and disable WiFi, this will stop the download of the Oreo Update and you have time to finish up.

Now you need to go to About and click 5 times on Build version to become a developer. Go to Settings -> Developer options and enable USB debugging. It

Connect the Nexus Player back to your computer (it should ask if you want to allow developer access, select yes) and do:

adb shell

Once in the shell type: su

It should pop up a dialog asking you to approve root, say yes.

Okay, now you are going to disable the otacerts.zip This will disallow updates to be applied.

Remount the system partition as readwrite

cd /

mount -o rw,remount /system

Disable otacerts.zip

cd /system/etc/security

mv otacerts.zip otacerts.disabled.zip

exit

exit

Okay, now your nexus player will no longer to able to apply updates. If you are happy here, you are good to go. The nexus player will still continue to try to download updates though so if you want, you should be able to disable it with this script: https://pastebin.com/E7QuvFdQ

Save as disupdate.sh on your computer

You put this script on your nexus player by using adb

adb push disupdate.sh /sdcard/disupdate.sh

You should then use adb shell again and move the script into place

adb shell

su (accept request on screen if it asks to become root)

mv /sdcard/disupdate.sh /sbin/.core/img/.core/service.d/disupdate.sh

chmod 755 /sbin/.core/img/.core/service.d/disupdate.sh

This will automatically disable the update service when the box boots. If there is no directory /sbin/.core/img/.core/service.d then Magisk isn't likely properly installed. You might need to reboot or something to get it to load.

That's it, enjoy Nougat, IMO the best running rom for the Nexus Player.