Hi psalm37wi,
Welcome to the Mattress Forum! 
You’re very welcome! Avoid the temptation to delve too deeply down the “rabbit hole” of information overload and paralysis analysis. I recommend to read through the information and use it as a reference and not attempt to memorize items, but learn just enough to determine if any manufacturers and retailers you’re considering are knowledgeable.
Yes, if you sprayed a poison on a natural substance, it could create an unhealthy combination. Luckily, that is not normally how mattress flammability guidelines are met domestically.
There is a lot of misleading information in the industry about fire retardants and “chemicals” ranging from significant “fear mongering” on one side to completely minimizing fire retardants as an issue at all on the other. Like most issues that arouse strong feelings or controversy the most reliable and “accurate” information tends to be in between both polar extremes so there is “some truth” that “some mattresses” may use some chemicals that some people would find questionable or wish to avoid. Some people may also wish avoid certain types of synthetic foams or fabrics as well even if they have been tested for harmful substances and VOC’s and would be considered “safe enough” by most people.
There are also many people in the industry that IMO exaggerate the risk (usually in an effort to sell some very costly mattresses) and seem to believe (or at least want their customers to believe) that every mattress except for an “organic” mattress (however they define this) is somehow “loaded with chemicals” to prevent them from catching fire which is far from the truth but this type of misinformation tends to scare people and lead people down a rabbit hole of conflicting and misleading information … and of course is exaggerated and somewhat ridiculous.
There is also more about “safe” fire barriers in this article and post #2 here and the posts it links to at the end and there is more about purchasing a prescription mattresses that don’t have a fire barrier and don’t pass the fire regulations in post #4 here and the posts it links to.
The most common method used to pass the regulations in the mattress industry is the use of inherent, “non chemical” fire barrier fabrics that are either quilted into the cover or are wrapped around the inner materials of the mattress like a sock. Wool is also used as a flame retardant barrier, most often in mattresses using latex or those promoting a more “natural” advertising agenda.
No, I would have no knowledge of the finances of all of the various certifying laboratories and certification companies worldwide. Depending upon the standard being tested (GOLS, GOTS, USDA, 16 CFR part 1632 or 1633, Greenguard, etc.), an independent laboratory is usually approved by the group holding the standard as qualified to properly test for whatever standard is in question. Many laboratories do testing for many different organizations on a variety of products. You could visit the website for whatever standard about which you were curious to see a list of approved testing groups/laboratories, or contact them directly for that information as well.
Your largest number of options will probably be found by looking online and using the experience and expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of latex and memory foam and other options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
Without an actual town, and subject to confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent ( see this article) and to making sure that any mattress you are considering meets the quality/value guidelines here … all I have investigated in your general vicinity are in and around the Minneapolis/St Paul area and are listed in post #2 here. If you are near Grand Rapids or Duluth then post #2 here may have some possibilities.
Phoenix