Hi fixfrozenpipes,
In most cases a mattress will soften and break down from the top layers down which are the layers that are most subject to the compression forces of sleeping on a mattress so it’s the upper layers where the quality and durability of the materials are most important not the deeper support materials or components. The deeper support layers aren’t generally the weakest link of a mattress in terms of durability. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
You are certainly heading in a good direction. As you probably know they are a member of the site which means that I believe they compete well with the best in the industry in terms of their quality, value, service, and transparency.
Your budget is certainly high enough to purchase a very high quality sleeping system.
Yes … two twin XL’s and a split king are the same thing.
This is certainly the “best” way to eliminate motion transfer. If you are also using separate mattress protectors and sheets then you would have no common points of contact at all where motion on one side of the mattress can affect the other side. There is more about the pros and cons of split king mattresses in post #8 here and in this topic
The choices aren’t quite as black and white as you’ve described them and there are many types of memory foam just like there are many types of every type of material and some types of memory foam are more breathable and have less effect on temperature than others than others and some memory foam mattresses and designs can also tend to sleep warmer than others. Where you are in the “oven to iceberg” range will also have a significant effect and I would keep in mind that while memory foam in general does tend to sleep warmer than other materials or types of mattresses … that only a minority of people sleep hot on memory foam (although they are probably a more vocal minority).
There is more about the different ways that can be used to “cool down” memory foam in post #6 here and there is also more about the many variables that can affect sleeping temperature in post #2 here and the posts it links to (including the mattress protector, any mattress pads, and the type of sheets and bedding you use which can have just as significant an effect on sleeping temperature as the type of foam in a mattress).
If sleeping temperature is your most important criteria then thinner layers of more breathable memory foam in combination with other types of softer and more breathable materials or considering other materials or types of foam completely that you find just as “comfortable” as memory foam may also be well worth considering. Your testing along with the guidance of a good manufacturer or retailer about the specifics of their mattresses will go a long way to helping you decide on which of the many options you have available (memory foam or otherwise) will be the best “match” for all of your criteria and the parts of your personal value equation that are most important to you.
I’m not sure how much this may help you because it’s unlikely that you would be able to find out all the detailed specifics of all the materials and components in the mattress you slept on to the degree that would be necessary to “match” it with another mattress because there are so many different versions of every type of material and component that it may have used. Even if you could find out all the information you need there may not be another mattress available here that would have exactly the same design and materials that would be necessary to “duplicate” it anyway. There is more about the different ways that one mattress can “match” another one in post #9 here but this type of “mattress matching” is very difficult and not particularly practical or effective except perhaps in the most general way.
Phoenix