Hi ColdfireTrilogy,
Your questions are much more complex than you may realize and go right to the heart of all mattress theory and design and material science which can take many years of study and/or experience to really become familiar with. They would involve a book rather than a post to answer in any meaningful “technical” way and even this would depend on the knowledge and experience of the person reading it to “translate” it into real life terms. There is a lot more detailed information in the various sections and pages of the mattresses section of the site as well as in posts like this one and this one (and others I could link) but I will add a few thoughts here to help you untangle some of this (at the risk of overwhelming you) and perhaps correcting some of the assumptions you are making that aren’t quite accurate.
I think it would be helpful first of all to think of a mattress as having 3 vertical layering “zones” rather than as individual layers that act alone. The layers themselves may make up part or all of one of these zones but every layer of a mattress will affect every other layer and every function of a mattress to differing degrees. They don’t act “discretely” in other words. The top zone is the pressure relieving zone and is the part that you sink IN and relieves pressure. This is most of what most people “feel” at least initially. The bottom zone is the support zone and these are the layers that “stop” your heavier areas from sinking in too far and sleeping out of alignment. This contributes to either producing or relieving “symptoms” that can come from sleeping out of alignment for too long over the course of the night and waking up with pain. The middle layer is more of a “transition zone” which both “helps” the pressure relief layers and as it compresses becomes firmer and helps with the support layers and the “feel” of the transition between the layers and the two main functions of a mattress.
The key is in understanding that these “zones” don’t have actual boundaries and all the layers of a mattress compress simultaneously together and some layers have a dual function. Even a 6" mattress with a single layer has these 3 “zones” because the initial compression is for pressure relief, the middle part compresses less and is partly for pressure relief and gets firmer as you sink in more and then the bottom part compresses least of all. This ability of most materials to get firmer as they are compressed more is called “compression modulus” an different materials become firmer with deeper compression faster than others. Even soft materials get firmer as you compress them more so as their more deeply compressed firmness becomes more than the surface firmness of the layer below it then the deeper layers starts to compress as well.
Thicker layers or mattress have a greater “range” from soft to firm (see post #14 here) which means they can be more adaptable for higher weights and sleeping positions but using different layers in terms of ILD or compression modulus can also compensate for this. All these different factors such as ILD, compression modulus, elasticity, thickness, and material type all work together simultaneously. In other words … “thick enough” (until differences are no longer perceptible) is the key and this may be different with different designs, weights, and sleeping positions. Beyond a certain point that varies with each person and their body type and sleeping positions … more thickness has little benefit.
The Beautiful, the Total latex Mattress and the Aloe Adelle either have or can have (based on your choices) a thicker softer top pressure relieving “zone” and the top section or “zone” that provides the pressure relief is most of what you “feel” at least initially. In your circumstances pressure relief would be very important. The “support zone” isn’t just the “left over” layers on the bottom though because alignment in all your sleeping positions is the goal and just like all the layers contribute to pressure relief to different degrees … all the layers also interact together to keep you in good alignment.
It certainly true that thinner mattresses will “bend” on an adjustable bed more exactly but different materials will also make a difference. Latex in particular is much more elastic and flexible than polyfoam and can also bend and adapt to adjustable beds more easily than polyfoam. This is especially true with the deeper firmer layers of a mattress.
Again … none of the layers are actually “dedicated” to only one purpose but all the layers will contribute different amounts to both pressure relief, alignment/support, and the overall “feel” of the mattress. the lighter you are the less the deeper layers will affect you or “contribute” to each of the other layers or functions of the mattress … with certain exceptions in certain types of designs (such as dominating layers which use softer foams under firmer foams where deeper layers can sometimes compress more than the upper layers).
Because the top layer has a significant effect on the overall feel and pressure relief of a mattress … which material you choose is important but it is also a preference rather than a better / worse choice. I would tend to narrow down your choice to whether you prefer memory foam top layers or latex top layers (or even a combination such as in the Aloe Adele) before I decided on the design. Of course it also helps when you are considering a mattress where you can’t test the specific combination to have options for exchanging or re-arranging layers in case your first choice doesn’t turn out to be your best one.
As a “translation” of some of this … The Beautiful has a 3" top layer of 21 ILD and then a 3" layer of 24 ILD and then 6" of 36 ILD (firm). Most of the pressure relief and “feel” would come from the top two layers. If you were to choose a mattress that had similar firmness in the top 6" over a firm support core (regardless of thickness to some degree depending on your weight) then the mattress would feel 'similar" to many people (assuming that the cover was also similar).
As you also mentioned … In addition to any layer exchange options you have you would also have the option of adding a topper to any mattress you chose if you wanted to use a different combination or to add extra thickness to your pressure relieving layers (softer material and thicker layers both add to softness in different ways).
As you can see … all of this can become overwhelming in its complexity and the truth is that most of the “fine complexities” won’t be as important as you may believe they are in real life … especially when you have options to change or add to them.
Now that I’ve probably overwhelmed you with “too much information” I would tend to first choose the type of materials you want in your comfort layer and then use your “educated intuition” after reading this (and I wouldn’t “study” it because it will only bring up even more complex information) along with discussions with the “experts” and your own insights from your personal testing to make a choice. All of your choices are good ones for different reasons and would have some initial differences and also different options to adjust or “fine tune” them after a purchase.
In your case I would work with what you know which is using the specs of the mattresses you have tested and liked (especially in the upper half of the mattress) as either a model or a purchase with the knowledge that you still have options afterwards if your actual experience doesn’t quite match what you need and prefer or what “theory says” as you are facing more difficult and complex challenges than most of us have to face which may also put you outside of the “averages” that are usually used to predict how a particular combination of mattress and person will interact together.
Now that I’ve said it would take a “book” it seems that I’ve almost written one and there is still so much that I’ve missed 
I hope this helps … and wasn’t “too much information” and if you have more questions or need more “clarity” about any of this feel free to post. You are certainly looking at some good choices that I think would be suitable for your circumstances both in their design and/or the options that they provide for fine tuning.
Phoenix