Fixed in the following way:
- installed LE NAND image [to NAND, obviously]
- rebooted the box with CoreELEC 9.2.0 microSD inserted
- selected to boot from SD by remote control
Now it boots from CE microSD properly.
Thanks to all who responded.
P.S. one issue which I noticed earlier still remains with this clean install - Update Channel list is empty as well as a list of available versions.