Hi sheep123,
Any density of polyfoam can be made in a wide range of firmness levels or ILD/IFD’s so a lower density foam could be anywhere between ultra soft or ultra firm and a higher density polyfoam can be in the same range. This is part of the chemical formulation of a foam which can create stiffer cells or softer cells. Density can play some role in firmness in some cases because higher density foams tend to have a higher compression modulus which means they get firmer faster as you compress them more deeply than higher density foams (so they can also be more “supportive”) but this isn’t always the case either because once again it depends on the chemical formulation of the foam. There is also more in post #4 here about the different specs that can interact together and affect how soft or firm a foam feels. Their old foam layers were about 2 lbs yes.
Fire retardant foams are not particularly common in mattresses which most often use barrier systems to pass the fire regulations. They are much more common in furniture.
Yes … heavier people would be more likely to experience this. There is more about the effect of thickness in post #14 here.
Yes … if you let them know you are a member here then you would be entitled to the 6.5% discount that they provide to the forum members here (unless there is a sale that is larger than the discount) and the trial period would also be extended to 60 days.
Phoenix