My posture definitely makes a big difference, I'm at 9 AHI sleeping on my side but 50+ on my back untreated. I'm gonna duct tape pillows to myself or something. Glad you found something that works for you!
I really hate that they do this. I had a friend who also got stuck with the same situation and did not have a good experience as a result. They should send you home to try the top-of-the-line model and then let you duke it out with your insurance company! ;)
Speaking side-sleeping, I have sewn a sock with a tennis ball to the back of my sleeping tshirt. It worked well, but it's hard to wash. So I bought this contraption: https://www.amazon.com/stream/ref=nav_upnav_mobile_T2_Detail...
Side-sleeping is a big deal, but it's even better with a neck pillow.
Also, check craigslist. In Seattle there are several people selling them right now at $400.
A good CPAP machine will tell you what your AHI is. Even the S8 does, though you have to get to the system menu (hold down and right arrows for five seconds).