The part you're missing is that multiple speakers don't receive the same signal in multi-channel film/tv most of the time. The center channel has most of the signal, and the rest of the speakers have less. Dialogue and main sound effects are almost always center channel. The other channels will vary entirely based on what directional effects are desired. So in any case there's no simple sum reinforcement that you can rely on unlike stereo or even multi-channel music to some extent.
Calibration is automatic by any decent AVR, Audyssey for example calibrates each channel to 75dB = -30dBFS. The sum of any channels is never considered at any point.
This is a common misconception that persists because of its simplicity. Sound only drops by 6dB in an empty field outdoors or an anechoic chamber. In a real room, it's more like 3-4dB. Toole explains
how to calculate this here. Of course, the exact amount varies, which is why you have to calibrate with a microphone in the first place and you can't just use the distance.
I'm confused what you're trying to show. Any good AVR with a mic will calibrate levels/distance to some standard. "90" on my Denon X4500H, when switched to the dB scale, would mean +10dB over reference for example. But without a calibration you have no way of representing anything. Reference/0dB is never maximum volume, that's usually about +20dB.
The only channel that's going to be an issue here is the center. The KH310 is capable of about 103-105dB anechoic at 10% distortion and 100-450hz. This is going to be your limiting range(or whatever lower bound for your sub crossover). Because it's not in a corner, it will only get half-space boundary gain, so you can raise that number to ~109dB(103+6). At 5m, that SPL will be reduced by around 6.75 - 10dB depending on your room characteristics. So 99-102dB will be reached at 10% THD, short of the 105dB peaks. This is why I said this will be fine for -5dB, or Dolby suggested loudness of 79-82dB. It is borderline above that. You will exceed 10% distortion at times, or potentially hit the limiter, I don't know when it kicks in.
This may be acceptable, it may not, that's up to you. It's certainly not a very severe limitation IMO but some people want to be able to hit reference with headroom and never have to think about any limits. For that, you would need a bigger speaker.
