Had to transfer extroot to another microSD by dd image copy. (SD died)
And seems new SDcard itself is another model or chip, but it works in a different way - I don't know, may be it is slower or for any other reasons... BUT - extroot needed additional time to be hooked during boot AND changing some parameters (like option_delay in fstab, export PREINIT=1 and mount_root in the PARENT system's rc.local) was needed. Made changes, now it's hooked, HOWEVER
Seems some services, init scripts, and some configs (like static host names) - don't start, I need to start them OR make simple changes manually.
I run scripts from rc.local myself, start services, re-order hostnames in LuCi DHCP section, then Save&Apply - and all goes well. Point is this is not as it was supposed to work.
PS I suspect, the main system during init has not enough time to see Extroot disk. So the system starts, but services, packages etc are NOT available in full at the paths where they're located. So boot switches to extroot but continues without them.