The gzipped libreelec image is burned very quickly because the uncompressed libreelec image is quite small (500 megabytes circa); the backup of the existing Android is the same of the whole NAND (8 gigabytes I guess).
Multitool is quite capable to handle gzip and other compressed formats: it will decompress them on the fly while burning is in progress, so no need to do that manually.
A possible issue that may come from your steps is the Flash Erase step you said you did. I don't remember if it is so, but it may be that the Erase Flash step completely erases the NAND, idbloader included. idbloader can't be installed again by multitool because of the NAND rockchip driver limitation and you may need to do that manually using rockchip upgrade_tool (see this post on armbian forum, paragraph
NAND Bootloader Upgrade)
I don't think I've removed the idbloader, I can reinstate the Android backup image using Multitool, and return everything back to as it was. Does that mean that idbloader is still intact?
Is there a good place to read up on that part uboot, idbloader and the kernel play in the boot process...