Hi katooom400,
I’m sorry to hear about all the difficulties you have encountered in your search for a suitable mattress. I can understand that it must be frustrating.
The only reliable way to assess the “safety” of a material in more general terms is based on any laboratory testing and certifications they have for harmful substances and VOC’s (see post #2 here for more information about some of the reliable “safety” certifications) and Tempurpedic is not CertiPUR certified. There are also some cases where people can have a sensitivity even to certified materials that most people would be fine with. Anecdotally Tempurpedic has the most complaints about their smell and sensitivities to their material (see here) but they are also the most widely sold memory foam mattress as well so some of the frequency of complaints would be the result of the sheer number of mattresses they sell.
All the major brands (such as Sealy/Stearns & Foster, Simmons, and Serta) tend to use lower quality materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (and the major retailers that focus on them) along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Like many of the members that are in a similar situation you are in a somewhat unfortunate position where you need to exchange a mattress and are “locked in” to a store that sells lower quality/value mattresses that I would normally suggest avoiding in the first place and where there may not be any particularly good quality/value options available to you.
Having said that … there are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation.
Since you already have a topper it may be worth exchanging for a firmer mattress (that feels similar to the Tempurpedic Flex you already have) with the “least possible” amount of lower quality or unknown materials and then using your topper on top of it.
Phoenix