Hi Pandabear,
Unfortunately your needs and medical conditions are far too complex for me to provide any specific advice based on what I call “theory at a distance” but perhaps I can at least help you with a step by step approach that can lead you to finding the best possible mattress for your specific needs and preferences. Bear in mind that a mattress may not solve some of the medical issues you are mentioning.
While it certainly appears like you’ve done some research on the site … the first step I would take would be to read post #1 here and especially any information it links to that you haven’t yet read. This will help you eliminate many of the worse choices and avoid the chain stores and mass market outlets (and possibly any “advice” they give as well although there may be some more knowledgeable people in some of them).
In addition to the links in the post (or the ones you haven’t read) … I would also read this article about airbeds before considering an airbed (which I believe would make a poor choice for almost anyone).
There is a fair bit of reading here but I believe it will be very helpful and even essential in your case before you start with the next steps which are identifying some of the better manufacturers or retailers in your area that carry the mattresses you want to test using the materials that you are most comfortable with. I also believe that personal testing on various mattresses will also be essential if for no other reason than to act as a guideline for an online purchase if your local options are more limited or not great value. I would certainly make local testing a priority though before taking the extra risk of an online purchase when your specific needs may be outside of the “averages” that most online purchases are based on.
It also seems that you are leaning towards latex which can be a good idea in your circumstances but the trick is to find the layering through personal testing that works best for you and unfortunately there is no “formula” that can choose your best possible choice that is more accurate than your wife’s personal experience and perceptions.
I think that zoning as you mentioned may also be helpful but may not be necessary with the right type of support layers (and Dunlop latex may be a good idea here because it can become firmer faster with compression and is more “supportive” than talalay in the same ILD.
From your description of the type of seat back she prefers it would seem to me that I would tend to avoid thicker comfort layers which may allow her to be closer to “firmer than average” support layers and sleep in a “flatter” position which may avoid a more “curved” or misaligned sleeping position where either parts of the spine are out of neutral alignment, where the midback is sinking in too far, or the shoulders are extended too far forward and sleeping in either a “hunched” or “shoulder slouched” position.
Once you have done the reading and preliminary research (and feel free to post any specific questions that you can’t find answers to along the way) … then it’s time to start testing mattresses and if you let me know the city or zip you live in I’d be happy to let you know about any of the better options I’m aware of in your area. Connecting with “experts” that have the knowledge and experience to give you good guidance and answer your specific questions about their mattresses and your specific needs is much more effective than trying to become an expert and “designing” your mattress yourself based on “theory”.
I’m not surprised a topper isn’t completely effective because if your mattress has softened then a topper is at best a partial or temporary solution because it will follow the curves of the softened foam below it. I would also avoid buying anything that is “application critical” from foam factory because what you receive is likely to be different from what you thought you were ordering (and your topper may not be Talalay latex or the firmness level you believe it is at all). There is more about them in post #2 here along with post #2 here.
So to recap … I would take one step at a time with some preliminary reading followed by identifying the best possible manufacturers or retailers in your area that have people with the experience and knowledge to help you and carry mattresses that you want to test and then testing mattresses with the help and guidance of an “expert”.
The knowledge and experience of the person you work with and buy from can be one of the most important parts of buying a mattress that is the best possible choice for your wife’s very specific needs.
Phoenix