
You've made my day.
My PW3 updated to 5.7.3.1 by accident last night, and I've managed to roll back to 5.6.5 following your steps.
Details:
1. I couldn't find "N" in my diags menu, but later I found sshd was already started, so I skipped step 5.
2. I kept all the databases in /var/local *untouched*, and fortunately it just works.
3. After the recovery, the jb bridged itself during the reboot, and my KUAL still works.
Also many thanks to @knc1, @NiLuJe, @ixtab, and those who help liberate
us from the darkness of the Amazon.