Hi Grace,
The first place I would start in case you haven’t read it yet is post #1 here. It has all the basic information, steps, and guidelines you will need to make the best possible choices.
You can read a little more about some of Essentia’s “claims” in this thread and this thread. As you can see what they say about their mattresses is suspect. It may also be worth reading this.
Yes … latex is the most durable of all the foam materials (polyfoam, memory foam, latex foam) and even in soft versions it is more durable than the equivalent softness in other materials. There are several manufacturers such as Flobeds that make component mattresses where individual layers can be re-arranged or exchanged either to make comfort choices or if one of the layers needs replacing so you can just replace a single layer rather than the entire mattress. A mattress and topper has some similar advantages because you can just replace the topper as necessary without having to replace the mattress underneath it but with a topper it can shift more than a layer inside a cover and it can also be difficult choosing the most suitable topper unless you can test the mattress and topper together. Some local manufacturers will also repair a mattress by opening it up and replacing a single layer. There is really no particular advantage to gluing the latex layers together in a mattress with a zip cover (see post #15 here). Overall a latex mattress is the longest lasting of all the mattress types regardless of whether it is soft or firm although with any material including latex … softer versions will not be as durable as firmer materials. There are many local manufacturers around the country that make great quality and value latex mattresses (both finished and with individual layers) that are exceptionally durable compared to any other types of mattresses.
Most latex mattresses do best on a firm non flexing foundation rather than a box spring which flexes. You can read more about foundations and boxsprings and which mattress each tends to do best with each in post #2 here and in post #3 here.
I’m not sure what your middle support layer is made of or how you planned to use it. If you can let me know how you planned to use it and what type of material it is and its softness and thickness I could certainly share my thoughts about it.
Of course if you get an adjustable bed then you won’t need either a foundation or a box spring.
There is no such thing as the “best” mattress or the “best” manufacturer because it depends entirely on your own personal value equation and on all the variables , tradeoffs, and personal criteria that are most important to each person. Some of the better options and possibilities in the Los Angeles area that I’m aware of though are listed in post #2 here.
Phoenix