How to set a different encrypted boot volume without unlocking?
I was able to change the default boot volume which is just a partial resolution of my question.  This is done by using an alternate form of the bless command based on an un-mounted storage device.  From the intended default volume OS booted:

$ sudo bless --device /dev/disk1s1 --setBoot

(where disk1s1 is the current and intended volume which can be observed from Disk Utility -> Info )
Now the OS that I want to be default boot boots by default.



Unfortunately,

I can't set the next OS to boot from the command line with running from the other partition.

$ sudo bless --device /dev/disk1s5 --setBoot --nextonly
Could not set boot device property: 0xe00002e2


Google search of the error shows similar challenges related to System Integrity Protection (SIP) but to me no clear resolution.