Hi charlesp210,
Just in case you haven’t read it yet … post #1 here is the best place to start your research and has all the basic information, steps and guidelines you will need to make the most suitable choices.
Almost all materials will have some “offgassing” to different degrees and an odor that goes with them as well (including organic materials) but this is not always connected with the “safety” of the material… Organic, natural, safe, and ecofriendly or “green” are all related to each other but are also very different subjects and one or more of these may be more or less important to each person. there are also a lot of information sources on the internet which confuse one or more of these with the others. There is a great deal more in post #2 here that can help each person answer the question of “how safe is safe enough for me?” and the related topics of natural, organic, and green manufacturing and what is most important to each person. These are all very complex issues and in most cases each person has to decide on where never ending research and in many cases no “black and white” answers ends so they can make choices based on their own “best judgement” and beliefs.
The number of layers in a latex mattress wouldn’t be a consideration for me in terms of the possibility of them shifting. Latex is very “sticky” and if for some reason the layers do end up shifting it 's an easy process to zip open the cover and “wave” them back into position (see post #15 here). Latex is also very elastic in all its versions and the firmness of the layers also wouldn’t be a factor for me if I was using it on an adjustable bed (outside of course of firmness being part of whether a mattress is suitable for your needs and preferences). In terms of thickness you will find that thicker mattresses will conform a little less exactly to the position of the bed but in mattresses up to about 12" or so this wouldn’t be a consideration either with latex.
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I’m thinking that I’d want 3 or fewer layers anyway. But some options could be a single 6" dunlop layer, a pair of 3" layers, or either of those with an additional 1-3 inch topper layer. A single 6" would be firm dunlop. With a pair of 3" layers, I think the bottom should firm dunlop and the top medium firm dunlop or firm or medium firm talalay (talalay being naturally somewhat more conforming)… With 3 layers, perhaps extra firm dunlop, firm dunlop, then the medium firm dunlop or firm talalay, I think 3 full 3 inch layers seems it would be too much for an adjustable, 2x 3 inch layers with one 1-2 inch topper layer would seem more appropriate. 3+3+1.5 in extra firm dunlop, firm dunlop, then medium firm dunlop (or talalay) on top sounds best to me.
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You can read a little more about the potential benefits vs the additional complexity of having more layers in post #2 here. You can also read ore about the two ways to make effective design and comfort choices in mattress firmness/comfort levels in post #2 here. The number of layers can make a difference in fine tuning a mattress for your specific needs and preferences but even a single layer may be suitable for some people and in this case there would be no benefit to having additional layers or options (and any extra cost involved) unless you wanted to make changes to your mattress down the road.
Post #21 here has the list of online manufacturers that are members of this site and two of these (Arizona Premium and SleepEz) allow you to choose between 100% natural or blended Talalay and 100% natural Dunlop or organic Dunlop. There are also others that offer two or three of these options as well. Of course there are some other companies that also have good options and good value that are not yet part of the membership here but the members here in their own way are certainly “among” the best quality/value in the country but of course this also depends on which criteria are most important to different people who define “value” based on their own personal value equation.
Phoenix