If you are not happy with the advantage plan, by all means, pursue the other options.
I have heard complaints about some of the advantage plans. I am still too young to be on Medicare but have daily worked a front desk and listen to the feedback about most of the advantage plans- and it is negative.
You may have been denied- does not mean you don't have to get the surgery- it would be on your tab!
(Expensive but that is why we have the ability to choose plans).

It sounds like the surgery will impact much of your spine. I would have ongoing conversations about the impact on your daily living. If you can maintain any range of motion in those areas, might be advantageous than the degree that you mention. In the meantime, while you work through to your best options, keep moving safely and follow your doctors' recommendations.