I'm not sure I understand completely but I was in a simliar circumstance. I had my surgery in three steps. The first was on COBRA, because my husband was laid off when his company was bought. That was through Aetna, because that's the insurance we had through his company. I was concerned that when he got a new job our new insurance wouldn't cover the second or third surgery, thinking they might consider it a pre-existing condition. He works for a Fortune 500 company with group insurance so that could have made a difference but our new insurance never questioned it once. I remember asking about it at my follow-up from surgery #1 at the Cleveland Clinic and they made it sound likes it's normally not an issue if you switch insurance.
After reading your post again I may have misunderstood your question?? Why would it take so long for your insurance to let you know if they will cover it? Usually, the hospital pre-authorizes all of that stuff as soon as they can to make sure they will be getting paid.