Hi Mr. Side.
Welcome to our Mattress Forum! 
I am very sorry to hear about the multiple health conditions. You certainly have some difficult challenges to deal with.
Having to deal with MCS and intolerances I can understand why you would make it your top priority to find a mattress that contains no off-gassing materials and why you would orient yourself towards more natural materials which would emit fewer VOC’s, (keep in mind that all mattresses would emit VOC’s to a certain extent, even fruit emits VOC’s). For those who are unusually sensitive … then choosing materials that don’t use harmful compounds can be particularly important. The “safest” of these materials are innersprings or microcoils, natural fibres (that aren’t treated in their manufacturing), rubberized coir, latex, and even densified polyester (see post #2 here ) or “soft solid” gel materials (see post #4 here ) although neither of the last two are natural materials. Most people who are more sensitive than the “norm” would probably choose to avoid polyfoam, memory foam, and synthetic fibers completely
The only reliable way to assess the “safety” of different materials in more general terms is based on lab tests and the certifications they have for harmful substances and VOCs so that you have some assurance that the VOCs are below the testing limits for the certification (see post #2 here for more information about some of the more reliable “safety” certifications). If the materials in a mattress or the mattress itself has a reliable “safety” certification then for most people they would certainly be “safe enough” … regardless of the type of material or the name of the manufacturer on the label.
In very general terms … softer and more pressure relieving materials that provide a more “relaxed” sleeping surface will tend to work better than firmer materials for most people with fibromyalgia where softer more pressure relieving sleeping surface is a more important priority, but I know that this does vary with individuals, as well as varying day to day. Latex in its softer choices would be a good choice as in general Latex does as good of a job at relieving pressure it is comparable to Memory Foam in its ability to form a cradle & form itself to the exact body shape. It has an unusual combination of surface softness and deeper firmness/support that comes from its elasticity, it’s point elasticity (ability to conform to the shape of a body) and its compression modulus (the ability to get firmer faster with deeper compression than other types of foam). This means that it can enhance the pressure relieving layers above it because of its surface softness and point elasticity but it is also very supportive and can “stop” the heavier parts of the body from sinking in too deeply. It is also very resilient (it returns a high percentage of the energy of compression instead of absorbing it like memory foam which has very low resilience) so it can enhance the “feel” of the mattress by making it more responsive and more adaptable to different body profiles and sleeping positions rather than the less responsive feeling of memory foam or the stiffer and less adaptable characteristics of polyfoam.
As you do have some very specific health considerations, nothing can replace your own personal testing. You’d obviously want to consider something that allows for enough surface comfort, and also assists with pressure point relief, while still overall being resilient enough and provide enough support while sleeping. Overall, the two basic functions of a mattress are to support and to provide comfort (you can read more about the basic functions of a mattress here ), with posture and alignment being the first priority and then comfort/ pressure relief “plushness” coming second.
A forum search on fibromyalgia (you can just click the link) will also bring up more comments and feedback from others that are in similar circumstances that may be helpful (there isn’t a single “best” combination of materials that will work for everyone with fibromyalgia and other compounded health issues).
Hopefully, this gives you enough information to help with your research.
I would be interested great to hear back from you to learn what you decided and I or any other Expert Member of the site would be glad assist you with any questions that you may have.
Phoenix