Hi Hystoryan,
3/4" of polyfoam in a quilting layer isn’t a durability issue.
The density of the latex would be a “comfort spec” and isn’t important (you can feel the softness/firmness of the latex with your testing).
The information you need to assess the quality of a mattress and the materials inside it is described in post #4 here.
An “all latex” mattress that only has 3/4" of polyfoam in a quilting layer wouldn’t have any weak links and would be a very durable mattress.
I would need to know the specifics of all the layers for the innerspring mattress you are considering to make any meaningful comments about the quality of the materials.
Phoenix