Hi lite1,
I think you may be overthinking things somewhat and the most common outcome of this is information overload and “paralysis by analysis” and in many cases outcomes that may not be what you expected. There are really only two ways to choose a mattress that has the best chance of being the best match for someone in terms of PPP. One of these is local testing and the other is a more detailed conversation on the phone with a knowledgeable and experienced online manufacturer/retailer (such as SleepEz) that has your best interests at heart and will know more about their own mattresses than anyone else (see mattress firmness/comfort levels in post #2 here). Of course a combination of the two is also a good idea when you are working with an online manufacturer. When you can’t test a specific mattress in person they will always be the best source of guidance about the mattresses they sell and the options they have available. There really are far to many unknowns, variables, and personal preferences involved to try and choose a mattress based on specs (either for a person or a mattress) or on “theory at a distance” without specific reference points based on personal experience.
I wouldn’t worry at all about whether a mattress is “progressive” or “differential” unless you are a mattress designer and even there these are just concepts and there is no clear “line” between one or the other and the real test is personal experience. Again I think this is too “theoretical” and technical to use as a guideline or basis for a choice. Either one can work well depending on the overall design of the mattress.
You’ve probably read this already but post #2 here has more information about the many factors that can affect the sleeping temperature of a mattress. For almost all people … a latex mattress with a wool quilted cover, a mattress protector made from natural fibers (such as wool and/or cotton), and cooler sheets (such as various viscose fabrics, cotton, or linen) will easily be “temperature regulating enough”. If you do decide to go in the direction of thicker layers of wool then I certainly agree that it can be a good idea to add them as a topper (which can be replaced without replacing the entire mattress) and there are many people who prefer to sleep on thicker layers of wool compared to any other type of sleeping surface or material but this is a personal preference and there are also tradeoffs involved because thicker wool layers will also affect the feel and performance of the latex underneath it. I would normally suggest starting with just the mattress and then deciding whether any additional toppers are necessary for temperature regulation or for preference reasons unless they have personal experience with a thicker wool topper over latex and know that they prefer how it feels and performs compared to sleeping closer to the latex itself.
I think that this would be a good plan and it will definitely provide a reference point for him to decide on his general preferences and whether he likes latex wool combinations better than just the wool quilting layer itself but I would keep in mind that any differences in the combinations you test vs the combination of materials that you end up purchasing can have unpredictable results. There are also differences between different types of wool toppers such as the type or breed of wool used, the amount of wool in the topper (oz/sq yd), how compressed the wool batting is, and how it is constructed and tufted, so one wool topper may not be exactly the same as the next even in the same thickness although it would certainly provide a good reference point for the combination of materials he prefers in general terms.
IMO … the “best” direction is a combination of personal testing that you can use as a reference point in combination with more detailed conversations with knowledgeable and experienced manufacturers or retailers that already know what you would otherwise have to learn. I would tend to avoid trying to “predesign” the “best” mattress or combination of materials. Even the “best” and most knowledgeable and experienced mattress designers are often surprised by the difference between what a particular combination of materials and components was “supposed” to feel like compared to how it feels in “real life”.
If after personal testing and more detailed conversations with the manufacturers or retailers of the mattress you are considering (such as SleepEz) you find you are still uncertain then the options you have after a purchase to change the comfort or support of the mattress, fine tune the design, or add additional materials or components would probably be a more effective approach and for most people would probably be a more important part of your personal value equation than trying to predict how a certain combination of materials may feel “in theory”.
Phoenix