Hi wh200,
It’s generally better to ask the manufacturer directly (and Brooklyn Bedding is very transparent about all the materials in their mattresses) but as far as I know it’s the same base foam as they use in the 13" gel memory foam mattress which is 1.5 lbs.
No … with polyfoam the density of the base layer has nothing to do with it’s IFD or softness … only its quality/durability and performance (density has a relationship to compression modulus for example which is how quickly a foam gets firmer as you compress it more). Any density of polyfoam can be made in any firmness level.
You can read about the many factors involved in durability (including the importance of foam density) in post #4 here. In most cases the upper layers are a bigger factor in the durability of a mattress (the upper layers are more subject to mechanical compression forces and softening or foam breakdown and are usually the “weak link” of a mattress) but if the upper layers are thinner then you will “go through” them more and then the quality/density of the layers underneath them will also play a more significant role in durability depending on your body type and sleeping style (which will determine how much you compress the deeper layers under the top 3").
The thinner top layer of memory foam will also mean that you will feel more of the firmness of the base layer below the memory foam which is why this is a firmer mattress than the 13" model even though the top 3" layer is the same.
Phoenix