Resolution, I don't know. I do have a a few more tidbits of information, though.
I could reproduce the "enter password" symptom by doing a "fastboot erase userdata" followed by a boot of the custom recovery. That is, by not having any filesystem at all, I could produce the symptom with TWRP - even though I have never encrypted the /data partition. So, at least certain versions (2.4.1.0?) of that custom recovery seems to regard a mount failure to mean that "your data paritition is encrypted" - that certainly is a "red herring".
iirc, there is supposed to be a
"crypto footer" in the last 16k bytes of the partition which lives outside of the end of the ext4 filesystem. I suppose that if you knew the exact size of your data partition, you could write zeros (dd if=/dev/zero ...) using the custom recovery to the last 16k of that partition and see if that clears anything up. Search the (above) link near the places where it mentions the "crypto footer". If that last 16k were all zeros, I would think that would cause "vold" to realize that there is no valid keys, etc in there. (But that's a little bit of a guess).
None of it explains why a device can sit on a shelf for months and suddenly start behaving differently on it's next boot, though. That smells a lot like bit rot or hardware failure in the eMMC flash memory.
But if the alternative is to throw the tablet out, it can't hurt to do a factory reset (stock devices) or a "fastboot erase userdata ; fastboot format userdata" (unlocked device). Both of those will nuke all data (including what's on the psuedo-SD card).
Anyhow, there's two ideas.
good luck