Good questions.
Yes it’s Dolby trueHD+atmos.(or DTSX)
How much are you missing? Not a LOT necessarily, but the juice is worth the squeeze. It’s definitely more immersive. As in the past, some tracks are much better and use the format better. My system is 7.3.4 and the experience is better in every way than when I was using 7.3.
Worth the $$$? Only you can decide but for ME, it was.
No. The front height, pl-IIZ will not replicate atmos. Wishful thinking. Lol
Also, DSU and dts NX upmix legacy tracks very nicely creating playback that can be almost as good as the real thing.
FWIW, IMO 5.x.4 is better than 7.x.2. It’s worth having four top speakers vs two, as long as you have some space between the LP and back wall.