Hi cabert,
Thanks for the feedback about your research so far 
In most cases I think these types of overly broad or “catch all” types of comments are usually true in some cases and not in others. It would really depend on the thickness of the quilting layers, the quality and durability of the quilting materials, and on the reason they were included in the mattress design. I would agree that thicker quilting layers that use lower quality materials can be a weak link in a mattress design which is why I would be very cautious if there is 2" or more of lower quality less durable materials in the upper layers of a mattress (quilting or comfort layers). There is more about quilting layers in post #12 here and the posts it links to. There is also more about the factors that can affect the durability and useful life of a mattress relative to each person in post #4 here.
I think the biggest “change” in foam is that many manufacturers are using lower quality versions in their mattresses than they did a decade or more ago.
I also agree that many people make it more complex than it needs to be and in it’s essence all that’s really necessary is to make sure a mattress is a good match in terms of PPP, make sure you know there are no weak links in a mattress in terms of the quality and durability of the materials, and make some meaningful comparisons between mattresses. Unfortunately in today’s industry with all the spin and marketing rhetoric, smoke and mirrors, and misleading information that is the “norm” in the mainstream industry … something that could be very simple and would be if all retailers and manufacturers were transparent, knowledgeable, and put their customer’s interests above their own, has become much more difficult than it really should be. Hopefully a growing number of more educated consumers can begin to turn the tide towards putting quality and value first once again.
If you are considering a “split” design with memory foam on one side only and latex on the other then I would give some thought to this and your sleeping habits because memory foam is a slow response material and latex (and polyfoam) is a fast response material that are very different from each other and the transition in the center of the mattress between them may feel somewhat “odd” … although one person’s odd may be another person’s “perfect”.
There are similar options available that would have thicker layers of memory foam or a similar design at a lower cost but they may not be available locally so it may depend on how comfortable you are with an online purchase. If you do consider this then the return policy may also be important in case the mattress doesn’t have the same subjective “feel” that you are hoping for (you can read more about matching one mattress to another in post #9 here).
This is also a “feel” that I like in certain combinations (in my case my preference is a fairly thin 1" or 2" layer of latex over a fairly thin layer of latex over a good support layer.
That’s a great spreadsheet … thanks
As far as which combination of memory foam would work best for you it would really be a matter of personal preference and the parts of your personal value equation were most important to you. It would also depend on the specific properties of the memory foam and the mattress as a whole and which one was a better “match” in terms of PPP. If you are in a higher weight range I would tend to reduce or minimize the use of 4 lb memory foam in the top layer of a mattress (4 lb memory foam would be more durable underneath another foam layer).
Yes … a mattress will tend to soften, compress, and break down from the top down which is why the quality/durability of the materials in the upper layers (which is usually the weak link of a mattress) is so important. The ability to switch out the layers either with a manufacturer that can open up a mattress and replace them or with a mattress that has a zip cover and loose layers that can be removed and exchanged means you can replace a single layer without having to replace the entire mattress.
It creates a different “feel” and a little more resilient sleeping surface but it’s thin enough that the memory foam underneath it will still provide the response and feel of memory foam overall. They will be happy to give you more information about the specific design of their mattress when you talk with them on the phone (which I would always do with any online purchase).
The cover is bamboo fabric and the quilting would be polyfoam. A quilted layer that is “around an inch or so or less” (and I would put 1.5" at the upper end of the range) would not generally be a weak link in a mattress. Once you are at about 2" or more then the quality of the quilting foam would become an issue and I would want to know the density of the polyfoam. I believe they use 1.8 lb density which is in the good quality range for polyfoam but they can confirm this.
The supersoft foam is at least 1.8 lb density so it wouldn’t be a durability issue and I believe they may have an option for higher density quilting foam as well. Again they can confirm this when you talk with them on the phone.
Phoenix