Pre-Authorized Out of Network Surgery and Hospital Stay, Still Waiting for Claim Processing

In late June, I had to do a surgery. The surgeon was in my network (Anthem/BC EPO in California), but both hospitals in which he has privileges were out of network. His office applied for pre-authorization and obtained one for the surgery and 2 days of hospital stay. The pre-authorization letter about the hospital stay explicitly stated that even though this hospital is out of network, this stay will be treated as in network as long as I stay during the time they approved and do the procedure they approved.
The hospital kept me for 2 days and 2 nights, and released me on the morning of the third day. During the second day, they asked me to sign a form that stated that I understand I'm not being admitted but I'm there for "observation". I have no family, so being alone in the hospital and not being able to do much research after the surgery, I just signed it.

As of today, the claim has been denied, stating that they are waiting for further information from the doctor.

My questions are:
1) Can they deny the surgery and the 2 days of the hospital stay, which they explicitly deemed medically necessary?
2) Can they deny the second night of the hospital stay? From what I understand now, insurance pre-authorization for 2 days isn't 2 periods of 24 hours. I don't know if the hospital obtained any further authorization while I was there and I wasn't in any situation to find out. I don't know if they were required to obtain one per California and Federal law either. What was the hospital's obligation and what is the insurance's obligation per California and Federal law?

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3) How can I speed up the processing of this claim so that it has a favorable resolution? The large hospital bill hanging above my neck has caused so much stress and anxiety in me that I could do without.

submitted by /u/backwardentropy