2. If you do not know how to make a copy of u-boot from a working SD card, you can ask the developers of the image for your TV box for the necessary file. It's not a big file "u-boot.bin.sd.bin". To record a working u-boot in two teams.
Note that the name mmcblk0 needs to be replaced with the name of your SD card in the system.
3. There are several important files that are involved in the startup process, which are used in your Armbian image, it is better to check with the image developer. You can try to copy all files from /boot.