I normally run CE but while ago I made some tests regarding the ethernet support in mainline LE release and I am planning to continue on this as soon as I have some spare time. At that time if coming from CE on SD, it was enough to reboot in android and redo one of the "boot from SD" methods (Reboot update, toothpick or whatever) to boot LE from the SD card (and again the same to eventually reboot in CE).
Now you write that it is required to restore the standard firmware. Are you talking about CE installed to internal, or it really affect any device that are currently booting CE from SD (and or USB)?
Coreelec are using an outdated and not compatible with other new systems technology run from external media. No matter what you have in eMMC. If you are running coreelec from external media, then your eMMC has an old version of multi-boot that is not compatible with the new systems. No new system can run properly on your device anymore.