My NAS540 is completely offline, after USB key upgrade

Where does this log come from? Do you know what the old firmware was, and what the new one?

Have you already tried a factory reset?

/mnt/partnerkey/buffer/DATA_a002
update upnp.db .....
/mnt/partnerkey/buffer/DATA_a002: line 8: sqlite3: not found
insert ssh info to upnp.db ....
/mnt/partnerkey/buffer/DATA_a002: line 14: sqlite3: not found

suggest that some database in config could be damaged due to a missing binary. Which suggests the the original firmware is incompatible with the post-update script in the .bin file.

When I compare that log to the flash script in that zipfile, a large part at the beginning is missing. Could be due to this forum, which tends to do strange things with computer generated data.

Anyway, the script advanced to it's normal end, so all flashing succeeded, apart from the config adaptions.

You failed to tell the old firmware version. If that was <5.10, there could be a problem with some disk content. Don't know the details, but somewhere I read that the 5.10 update had some special scripts to convert certain disk structures. You can pull the disks, and see if it then boots.