1) You are correct in that the official Oreo OTA requires either the Feb 2018 security patch Nougat 7.1.1 firmware or the most recent leaked Oreo soak test. The most straightforward way would be to download the Feb 2018 stock firmware and flash that via fastboot. You do not have to re-lock your bootloader to flash the stock ROM, but I would recommend you take a backup of your device in TWRP and keep the backup safe, in case the flash goes wrong.
Here's a link for the fastboot stock ROM for the Feb 2018 stock firmware:
https://forum.xda-developers.com/showpost.php?p=76180201&postcount=21
Flashing commands and the guide for setting up your computer with ADB and fastboot can be found here
https://forum.xda-developers.com/moto-z-play/how-to/guide-relock-bootloader-to-lasted-stock-t3718190 If you do not want to re-lock your bootloader, you can omit the second set of commands and if you wish to keep your data, you can try and omit
fastboot erase userdata though you may wish to include the erase userdata command for a clean flash.
Thus, if you wish to simply revert back to stock with the Feb 2018 firmware - no bootloader locking, and try to keep your data - you could use these commands (though I have used the same set of flashing commands in the ADB terminal to flash stock firmware, I do not own a Moto Z Play - I have a G4 Plus, so you may wish to wait for a Z Play owner's input. These are however standard fastboot flashing commands for Motorola stock firmware).
Code:
fastboot getvar max-sparse-size
fastboot oem fb_mode_set
fastboot flash partition gpt.bin
fastboot flash bootloader bootloader.img
fastboot flash modem NON-HLOS.bin
fastboot flash fsg fsg.mbn
fastboot erase modemst1
fastboot erase modemst2
fastboot flash dsp adspso.bin
fastboot flash logo logo.bin
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img_sparsechunk.0
fastboot flash system system.img_sparsechunk.1
fastboot flash system system.img_sparsechunk.2
fastboot flash system system.img_sparsechunk.3
fastboot flash system system.img_sparsechunk.4
fastboot flash system system.img_sparsechunk.5
fastboot flash system system.img_sparsechunk.6
fastboot flash system system.img_sparsechunk.7
fastboot flash system system.img_sparsechunk.8
fastboot flash system system.img_sparsechunk.9
fastboot flash system system.img_sparsechunk.10
fastboot flash system system.img_sparsechunk.11
fastboot flash system system.img_sparsechunk.12
fastboot flash oem oem.img
fastboot erase carrier
fastboot erase cache
fastboot erase DDR
fastboot oem fb_mode_clear
1a)You may wish to verify that the number of system.img sparsechunks is the same in the above command list as in the unpacked firmware folder.
1b)Be aware this procedure will replace TWRP with stock recovery, remove root and any modifications you've made. If you are not happy with that, then you may wish to wait for the Oreo TWRP flashable stock ROM.
1c)This procedure should update your bootloader to the Feb 2018 C1:14 bootloader. Please do not downgrade your device's stock firmware to older stock versions if you can avoid this - or if you do downgrade, in general do not use OTA updates. This warning also includes attempting to downgrade to the Dec 2017 firmware to use the Oreo soak test OTA - do not use the Oreo soak OTA if you have downgraded from the Feb 2018 stock bootloader. Downgrading your stock firmware is not usually the issue, but doing so leaves your device with a newer bootloader but older system (.e.g Feb 2018 bootloader, Dec 2017 system firmware). This downgrade mismatch between the bootloader and system is what can lead to hard bricks, particularly if the OTA-supplied bootloader and your device's bootloader are substantially different (eg. the OTA attempts to flash a much older bootloader or a different bootloader over your newer bootloader - OTA updates do not appear to verify your bootloader version, they just assume your bootloader is the same version as your system firmware). The blankflashes we have do not appear to rescue devices bricked with previously flashed with the newest bootloaders, so be careful if you do downgrade your stock firmware. If you see yourself downgrading in the future, you may wish to wait for the TWRP flashables, which do not usually affect your bootloader version. Only full stock ROMs and stock OTA updates care about the bootloader version.
1d)After flashing the Feb 2018 firmware, your device should be booting. However, if you encounter any instabilities, you may have to factory reset your device. Depending on how different the TWRP flashable of the Dec 2017 firmware you have is and the stock firmware, you may not see any instability, but that's why the TWRP backup before you flashed the stock firmware is there, in case you have to erase your data.
1e)Then, you should be able to sideload the Oreo OTA or if you're somewhere where the OTA has been deployed, you should see a notification to download. It seems that the OTA will take some time to install and appears to stall at certain points, so you may wish to allow plenty of time for the flash. Also, ensure your device is powered or has plenty of battery whilst the flash is proceeding. Also, I think we do not currently have a fastboot Oreo stock ROM, so if you choose to root or modify your device after flashing Oreo, ensure you have a backup.
2)You could wait for the TWRP flashable if that's easier - as I mentioned above, you would not have to worry about your bootloader version since TWRP flashables don't affect the bootloader (though I recall some ROMs have a bootloader requirement) and may allow you to keep TWRP on your device. Hopefully there's an Oreo TWRP flashable coming along.
