Hi smmimp14,
I have to agree with you about the poor state of knowledge possessed by most mattress salespeople about their products. Sometimes this is due to the manufacturer not sharing meaningful details about their products (as in this case), and sometimes it is from the philosophy of the store ownership prioritizing sales tactics over knowledge. And sometimes it is both things.
Unfortunately, without knowing the thickness and density of the materials within any mattress, there is no way to determine the quality and durability of the product – only the initial comfort when testing it out.
Regarding your weight comments, you are correct that someone with a higher BMI will sink into a mattress more and compress materials more as well, especially in the uppermost layers. Post #3 here has more information and suggestions about higher BMIs that you may find interesting.
Phoenix