Hi JaniceS,
Welcome to the Mattress Forum! 
I’m sorry that your Beautyrest and now your Tempurpedic mattresses haven’t worked out as well as you had hoped. Maybe I can offer some advice.
First, regarding the use of a power foundation while sleeping upon your side, some people do find is comfortable with a slight head, and even foot, elevation, but the degree to which that can be adjusted will be limited by your anatomy and flexibility.
As for your Tempurpedic Cloud (you didn’t mention which model) feeling firmer as the night goes on, this would be somewhat normal for a visco elastic memory foam product. As your body heat softens the foam throughout the night (the memory foam becomes less viscous), you will tend to sleep in a bit more and then “feel through” the memory foam and be exposed more to the comfort of the firmer polyfoam core of the mattress. This could be the reason behind the soreness you’re feeling on your hips, back and shoulders, which is common .
Using a topper can help to alleviate some of this sinking and also provide a bit or surface resilience and breathability when the topper is latex.
I personally like relatively thin layers of latex over relatively thin layers of memory foam because it can provide a more resilient and “movement friendly” surface feel and performance to the mattress while at the same time you can feel yourself more slowly sinking into the softer memory foam underneath. The latex would isolate the memory from your body heat to some extent so it would respond more slowly and/or be firmer than if you were sleeping directly on the memory foam. This can create a very interesting “feel” that some people like.
There was specifically a very popular mattress that was made a few years ago that used high quality memory foam similar to your mattress, and they included a 2" layer of 19 ILD Talalay latex as the uppermost layer within the product. By placing the latex on top there was a bit better breathability within the product on top, an increase in the “buoyancy” of the feel of the mattress, but there was still the dense memory foam feel beneath. Of course, this is a general statement about the product based upon quite a few people with whom I’ve spoken, but your thoughts are certainly not out of line and your choice of componentry would be, “at a distance”, appropriate to achieve what you’re describing. You are correct that some of the “sinking” of the “heat activation” of the memory foam will be minimized, but it will still soften a bit. Using a slightly "softer latex (let’s say in the 19-24 ILD range) will provide some extra plushness, and in the upper 20s could provide some comfort with a bit of extra surface resiliency.
Between the Dunlop and Talalay latex you mentioned, the Talalay tends to be the more “buoyant” and “springy” feeling, and for toppers overall the most popular.
Ther is more about purchasing a topper in post #2 here, and it links to posts with topper sources, including some that can be returned.
Post #2 here speaks more specifically to latex toppers.
Post #8 here talks about selecting a topper.
I hope that information is assistive to you.
Phoenix