I did read the articles you posted and I do understand that the situation is very "complex". This is exactly why I built the room in such a way that nothing is necessarily permanent in terms of room treatments.
There is no inherent guarantee that any design by any designer will necessarily sound perfect. In fact, I'm not really sure there is such a thing as perfect. I literally guessed at some of the surfaces based on pictures from various acoustics websites.
Having been through a ridiculous amount of acoustics articles, forums, user comments, pictures, acoustic products websites, etc... I have come to the conclusion that there are no absolutes. I have seen articles about diffusion behind the listening position that recommend it and others that do not (for example). Sitting too close to some types of diffusers make them not function properly as well. There is no way in the universe I was going to know what exact diffusers would work best, what scale they should be built in, or how it would affect specific listeners in the room.
I knew going into this that I was almost certain to like the sound of an arbitrarily arranged diffusive room with plenty of bass traps compared to a simple rectangular room. I certainly don't like an echo room and I definitely don't like a dead room either. Something in between albeit imperfect ... is ok by me.
I also knew that I wasn't going to get enough of an interesting room architecturally if I simply went with a "home theater designer". Since the room is somewhat modular, I can actually take out the hemifusers and do QRD's or something else based on future budgets for modifications.
There is simply no mathematical computer in the world that is going to accurately model what the diffusion/absorption characteristics are going to be until I get the whole thing running and test it in reality. There are SO many bumpy surfaces and trim of different sizes all over the room. The ceiling is not flat either. Once the furniture/rug/people are added in, there will of course be additional absorption.
Like you said, some of the diffusers may actually demonstrate various levels of absorption. Who can really say in such a complex room what the final results are going to be? As a person with reasonable scientific degrees... I know that there are enough variables in this equation to make "perfection" unknowable a-priori. I'm just going to listen to the whole mess and pray it comes together the first time... If not, time to play with the treatment arrangement a bit with the help of some acoustic measurements.
So far, spoken words in the room sound extremely clear with a very "live" sound without any hint of echo. The noise floor in the room is very low also but I don't have the proper equipment to test the actual numbers yet. Based on my current home as reference (~20db), I would estimate about 15db of ambient noise. With the AC off and when there is no wind outside, It may be closer to 10db noise floor.
Part of the room that is not done yet is the stone on the wall around the base of the room. Each piece of stone will be bumpy and will actually be placed at different depths just like the outside of my house. This will add additional differences in acoustics.
I would imagine that most people simply do not put in this much diffusion and architectural surfaces to really know one way or another how it might sound. It's certainly a fun experiment for me and I will certainly report back if the whole thing sounds like a mess or what.