Unfortunately, as of the locked state, the phone refuses to boot into TWRP using
fastboot boot <twrp-image-file>.
As you suggested, I tried switching to the other slot (a):
fastboot --set-active=a
which results in the phone rebooting directly into the “data corrupt / try again/factory reset” screen, so I switched it back to slot b.
In the above “TWRP for Faiphone 3” topic @k4y0z said:
How exactly to do this?
Sure, I still have my pin 
.
I guessed from the fastboot help output and tried fastboot fetch userdata ./userdata.img, but the phone answers, that fetch is not supported on this phone.
Another idea:
Would it help to again flash the latest OTA-Update with sideload mode?