Regarding the disc level, and which is it. Your very bottom disc should be between the vertebraes L5 and S1. Is the disc they are referring to the very last or bottom disc in your spine?
If this is the case, this would again be the best area (if there is a best, opinions vary) to have a fusion. There are no discs below it to add pressure on and the flexibility there is less than other areas, from my understanding. Most insurance companies are still denying ADR, although that doesn't mean they won't start approving it any day now. Fusion should be fully covered by any insurance.
This is the part of the decision that we all agonize over, to wait for insurance to pay or to have doctors become more experienced, to take fusion now? Is fusion ever good? I would jump on the same bandwagon a few others are on and say get multiple opinions from doctors who you know are experienced with both surgeries before you make a decision.
My doctor is Dr. Bitan, in NYC, he was involved in the charite trials over here and had some experience in Paris before coming to the states. He was the one who gave me the diagnosis that made the most sense to me. I saw three different reputable doctors back to back before I made a decision. Just another avenue for you to explore!
Good Luck!