I wanted to follow up here with closure. Im not exactly sure WHY what I did worked, but it did, and maybe it can help someone else.
last I left off, I had CM11 installed, but no wifi, and the partition was still encrypted. At this point, I decided to try to downgrade again to CM10.2
Now, when the tablet was booting, it was asking me for my password to decrypt so it can boot. Except now, after loading CM11 and going back to 10.2, the password wasn't working and couldn't boot. I was convinced I was bricked.
I went back into recovery, and reloaded CM11. This time, when it came up asking for my password to decrypt before booting, it worked. So something in CM11 changed the encryption on /data so that CM10.2 couldn't read it anymore.
once I was booted into CM11 (still with no wifi), I went to SETTINGS - BACKUP & RESET - FACTORY DATA RESET again. This time, it worked! Asked for my password, said are you sure, then rebooted. BOOM, /data was no longer encrypted! CM11 came up, and with the fresh /data partition, wifi even worked!
For other reasons (Netflix) Ive since gone back to 10.1, but now /data isn't encrypted, so wiping and flashing is easy and works great now.
It seems there is a bug in CM10.2 (or android) where the FACTORY DATA WIPE doesn't work with encrypted /data partitions, but CM11 was able to do that just fine.