So, I managed to get the device working after all! I ended up doing these steps:
1. Powering down altogether with all 4 drives in bays.
2. Attached the TS to my main computer via network cable.
3. Opened TFTP boot.
4. Booted on the TS; got the usual E06/blinking error. (But all 4 drives showed green lights.)
5. TFTP served the two boot firmware files. Eventually, TS stopped erroring out and gave a "Welcome" message.
6. Reattached my TS to the network. Eventually it got a 192.168... IP.
7. Opened the 1.71 firmware updated on my main computer. Debug mode with all boxes ticked.
8. Hit Update and the format FINALLY worked!
9. TS did a bunch of reboots and eventually booted properly. No errors. All drives recognized.
10. Opened NAS Navigator, signed into the admin GUI, encountered the weird Japanese language bug. Changed to English.
Since then, I built a raid 5 array, shared my folder, set up the DLNA server, and I'm now off to the races.
Thought I'd post this in case it helps anyone. Also, really appreciate the help from davo I received on here to get me going. Also, this guide was instrumental in piece it all together:
https://reddit.com/r/sysadmin/comments/74xoju/how_to_fix_a_buffalo_terastation_tsxel_in_tftp/. Sequence matters. Patience (which was tested a few times) matters. The willingness to start over matters. Feels like a Buffalo mantra.