The fact that the % dropped so much seems to indicate that the contactors are closed! I believe their entire purpose is to completely isolate the HV battery when open. If they're closed, you might be able to get OBD to work.
When you disconnect the 12V, is there any voltage at the CABLES, being supplied by the car?
Assuming you tried "only":
- HV disconnect/reconnect with the 12V disconnected the whole time, then reconnected.
I would still try:
- Another 12V disconnect now.
- As NHTSA says: HV disconnect with the 12V connected, THEN disconnect 12V, reconnect HV, then reconnect 12V.
- As ratfinkharley did: HV disconnect/reconnect with the 12V connected, THEN disconnect/reconnect 12V.
If all that fails & nobody has any other ideas, I hope you've found a nearby service center with some knowledge. I hear some Bosch centers can help.
In case you'd planned on reconnecting the humidity sensor, I wouldn't bother. I unplugged mine about 5 years ago when it kept deactivating recirculate, & activating my A/C compressor without even lighting the A/C button! I've had it at the Fiat dealer for at least one software update since then & they didn't seem to notice.