Q2: How to downgrade bootloader in case I want to roll back without unlocked bootloader?
A2: Rename existing Android 8.0 OTA package for your model to D1C-9999-0-00CN-B09-update.zip. Then you should know what to do next.
Android 8.0 for TA-1000 DL: https://ota-filesite.c2dms.com/SWImage/500000772 (DO NOT USE IT ON NON TA-1000! )