Just a suggestion: Could you be over thinking this? Some thoughts
- There are simple tests using the Chung approach you can do yourself in the real world. For me, in none ideal circumstances I was able to pick up the difference between a shrugged and non-shrgged head position (and different front wheels). (Look up Chung, then use Golden Cheetah.). In other words, stop over-thinking it and go and do some real world experiments would be my suggestion.
- There are a whole host of assumptions in your long introduction (Shoulder positions and helmets, smoothness, , gaps etc.). Are they actually true? How do you know?
- On the minor issue of the Specialized, I though the “gap at the back” was to let air out and smooth the flow, so it is more aerodynamic.
- I do TTs from 10m to 12hrs. I am a very average TTer, though had a few good results for my age and ability and am still learning. I demand comfort and aerodynamics from my helmet. I am very happy with a Giro Aerohead. Teammates have the Kask Mistral and I could have had one at a great price, but choose not to. The reason.: The giro was probably fine and ther was much much more I could do with my body and power and fitness and nutrition during races, that would give me more bang per buck.
- Sometimes helmets come down to comfort and fit. My aerohead often gives a high pitched whistle/reverberation on dual carriageways with concrete surfaces and tyre noise. So I simply wear ear plugs to mitigate it and can still hear the traffic around me. I like that the visor parks easily. In a long race I could park it, then refit it if it rains. The point I am making is we are not pros and sometimes a simple compromise solves a problem.
- Can you hold your head in a position that still looks up the road, is fine for your shoudlers and arms and is still areo. its a personal thing. My neck has some age related stiffness. Yours might be a different shape. If you are constantly looking down, that is as much a physical issue as a helment issue. (The helmet tail pointing skywards just makes it worse).
There is a bigger piece here. These pros are all on different bikes with different arm positions. Each one works for them. Frankly, I doubt there is a big real difference amongst bikes. Can you get in the position on them, counts for much more. And they are far far more flexible than most of us. I think personal felxibility (esp, around glutes and hamstrings and neck) can make a massive difference.
Almost finally - I had a TT bike fit recently. Looking at my positions before and after, they look very similar. Actually they feel dramatically different and afterwards is much faster. Flexibility played a big part. Also there is a whole thread on Facebook of people criticing other people’s positions from a picture. Frankly, I really do not think you can tell as much, as people make out, and they assume so much.
Finally, I am facsinated that your LBS allows you to sweat in a helmet to try it out… and then return it… (Remind me not to use them… :))
Good luck with this. (A practical TT cynic 
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