Hi jbzzz,
The combination of all of the materials will contribute to your overall comfort, but in the Sealy Hybrid you mentioned there is a zoning of 30% more pocketed spring in the center section of the mattress, along with extra padding in the middle third of the mattress.
I’m guessing what you mean by “petroleum” is polyfoam or memory foam? This is technically a combination of polyols, isocyanates and water, in its simplest form. I think you mean you’re attempting to minimize the precursor source of the polyols used, which can be from petroleum sources. This would leave you with more “natural” materials such as natural latex or other natural fibers and springs.
As for flame retardants, fire retardant chemicals have never been mandatory. What is required is that a mattress pass the 16 CFR 1632 and 1633 fire regulations with or without the use of fire retardant chemicals (the method of passing the regulations isn’t specified in the regulations). The most common method used to pass the regulations is the use of inherent fire barrier fabrics that are either quilted into the cover or are wrapped around the inner materials of the mattress like a sock. There is more information about fire retardant methods that are commonly used in the industry in post #2 here and in post #4 here.
If you’re looking for more “natural” materials, then yes, springs, natural fibers (cotton, wool, silk, kapok…) and latex will tend to be what populates in your search results. A search on just “zoned innerspring” brings up hundreds of results, but I don’t know exactly what combination you may wish to pursue, because you like the feel of memory foam but wish to avoid it for reasons of precursor content, but selecting latex seems to not be high on your list due to previous results.
I don’t keep a listing of the tens of thousands of offerings for mattresses that are available online and in retail locations, as it would be much too large of a task for any one person to successfully undertake in an ever-changing market.
One thing that immediately comes to mind is that perhaps you’d like to try a slow-recovery latex, such as Puralux. It will have the “feel” of a memory foam, but perhaps a content with which you are more comfortable. You could then combine this with a zoned innerspring unit like the Leggett and Platt Combi-Zone, and perhaps this would be a solution. Many of the site members listed in post #21 here also use zoned innerspring units, but most in combination with latex.
Additionally, you may wish to look at a mattress using no foam. Post #4 has a list of such items, and while it is not a complete list (and I’m not aware which companies may be using a zoned spring unit – you’d have to check on that), it would be a good start.
While I know the information I’ve provided is a bit general, hopefully it can assist you in narrowing your focus and then maybe I can be more specific with any questions you may have.
Phoenix