Hi brosen2,
I think you have a good handle on the many preferences, choices, and tradeoffs involved in deciding on a mattress that is the “best value” for you but I can make some suggestions that may help with “how” to choose even though only you can decide on “what” to choose.
Post #13 here has more about the most important parts of the “value” of a mattress purchase that may be helpful and may give you some different ways to look at “value”.
I would keep in mind that there are different types of support and that support isn’t just related to firmness and that the goal of a suitable mattress is to keep you in good alignment in all your sleeping positions. A mattress that is too firm (or sleeping on the floor as a more extreme example of the firmest sleeping surface possible) would provide little to no support under the recessed parts of the body (such as the waist or the small of the back) and sleeping on a mattress that has comfort layers that are too thick and soft or support layers that are too soft may do a good job “filling in” the recessed gaps in your body profile but may allow have poor support under the heavier hips/pelvis and allow them to sink down too far which would also result in poor alignment. There is more about what I call primary and secondary support and their relationship to pressure relief (which are the two main functions of a mattress) in post #4 here that may be helpful to understand the differences between “support” and alignment.
There are too many unknowns, variables, and personal preferences involved for anyone else to know which mattress is the best match for you in terms of PPP based on specs (either yours or a mattress) or “theory at a distance” and the only reliable way to assess this is based on your own personal testing (using the testing guidelines in the tutorial post) or your actual sleeping experience (see mattress firmness/comfort levels in post #2 here). Latex has a higher compression modulus (the rate that a foam material becomes firmer with deeper compression) than other types of foam and it also has a high “point elasticity” (the ability to contour to the shape of the body) which are some of the reasons that latex is somewhat unique because it can be both soft and supportive at the same time so the odds are higher that even softer versions of latex would be “supportive enough” compared to other types of foam materials but again your own testing is the only way to know this.
post #33 hereA firmer layer over a softer layer is called a “dominating” layer and is a preference choice that some people prefer. There would be no durability issues with this layering (latex is a very durable material in any combination and a soft layer under another layer would be more durable than a soft layer on the top of a mattress because it would be subject to less compression forces). There is more about dominating layers in and the posts it links to.
Outside of the thread you linked … posts #3 and #4 here has more information about the differences between a separate topper and having the same layer inside the mattress cover. Again though … your own testing and experience will be the only way to know which one is best for you. The separate topper would act more independently and feel softer so it would tend to have less primary support and more secondary support.
I agree that it most people would find the price differences hard to justify when the mattresses are so similar but there may still be some people that for reasons of their own would still consider the Savvy Rest to be better "value. I would also keep in mind that SleepEZ also has a good return and refund policy and Savvy Rest just has an exchange policy. In the end though … when you are down to final choices between “good and good” (see post #2 here) then it will come down to “best judgement” and preferences based on all of the objective, subjective, and intangible parts of your personal value equation that are most important to you.
Phoenix
PS: Your questions have almost turned this post into a mini course on mattress design and theory 