Hi Theologator,
Fox mattress is one of the members of this site and like all the members here I invited them because I believe they are among the “best of the best” in North America. I think very highly of Rick and his ability to help his customers find a mattress that is best suited to their needs and preferences and of course they have great quality and value. You would do well to work with him to choose a mattress that works well for both of you.
Some of your better choices in the general Orlando area are in post #2 here.
I think your criteria are good ones.
In your situation I would place a lot of value on a local purchase … especially when there are such good choices near you. In person help and guidance that comes from someone that has the knowledge and experience to help you choose a mattress that fits your more unusual needs and preferences would be particularly valuable IMO.
Bayer actually produces the chemical “packages” that are used by the foam manufacturers to make foam and the foam itself comes from an American foam manufacturer. It could be NCFI but I’m not sure which specific foam manufacturer they use although I know it’s American. I would consider all the American foam manufacturers to be “safe” foam. Most of them are members of the voluntary CertiPur program although many mattress manufacturers who use their foams have not yet become part of it or may not promote the fact that they are using CertiPur certified foams. the acceptance of CertiPur among mattress manufacturers is increasing even though there is a cost involved in becoming part of the program.
This is a reasonable budget for memory foam and should include good quality materials (although not necessarily the best quality) in your mattress although this would also depend on the size you are considering because of course there is a significant difference in the cost of different sizes.
How long a 4 lb memory foam will last with someone over 200 lbs would depend a lot on many factors including the layers above and below it and perhaps most important of all how sensitive you are to any softening of the foam in your mattress. If you buy a mattress that is already on the soft side and you are on the border of the range that meets your needs and preferences, then any further initial or ongoing softening would put you on the other side of your ideal range of pressure relief and support. If you start off with a mattress that still has some “room” to soften and is more in the middle or upper end of your range … then the initial or ongoing softening will still leave you in the acceptable range for your needs and preferences. In general … thinner layers of 4 lb foam in combination with 5 lb foam would be more durable for someone over @200 lbs but this also depends on other factors. There is more about the different factors involved in mattress durability in post #2 here.
You have some great choices within reasonable driving distance 
Phoenix