the reason it show Google Play Protect not be certified is Google does not require that machines sold overseas must be GMS certified until Android 8.0.
and about your 3 qusetions:
1.using mtk spflashtool to flash factory firmware do not need unlocking bootloader.
2.this factory firmware flashing need you Paying attention to the backup of the original data, because it tends to erase your machine’s data.so don’t flash system.img alone
4.MTK platform machine normal flashing will not let the machine die completely, even if it looks like a fake dead state, try to make the machine completely power off it will can be re-flashed
