Hi amueller,
I don’t know the ratings of the layers in the Renew but they would be in the N1 - N5 range and you can see the ILD range of each of the “N” ratings here.
If I had to guess I would say the top layer is N1 and the middle layer is N2 and then the base layer would either N4 or N5. The “all natural” toppers would be N1. If you call PLB they may be able to give you more specific information (and I wouldn’t consider these as more than guesses).
No … the blended is made to a more specific ILD target (they average 9 measurements across the surface and then assign the ILD that the average is closest to based on the ILD ratings for Blended Talalay here. Each measurement may be +/- 2 from the rating. The “all natural” is only made to an ILD range and doesn’t have a specific ILD rating. 100% natural is also a little more dense than the blended Talalay so it will get firmer a little faster as you compress it more deeply (has a higher compression modulus). There is more about the difference between blended and “100% natural” Talalay in post #2 here.
With an appropriate quilted cover the split itself wouldn’t be noticeable but you would notice the difference in firmness between the two sides if you slept in the middle. There is more about split layering in post #2 here. Softer layers will soften and break down faster than firmer layers in any foam material and the softness of a material is one of the secondary factors in form durability (see post #4 here).
That depends on the design goal of the mattress and the rest of the layering and components and how they interact with your specific body type and sleeping position. Softer base layers with thinner comfort layers will be “roughly equivalent” to firmer base layers with thicker comfort layers in the same ILD. ILD, compression modulus, point elasticity, resilience, and layer thickness all interact with each other to create the “feel” and performance of a mattress and every layer affects every other layer to different degrees. There is more about the different specs involved in the “feel” of a mattress in post #2 here.
In general if you are trying to “'approximate” or even “duplicate” another mattress it’s best to use the exact same materials in the same thickness and ILD and a similar type of cover because every difference between your test mattress and the mattress you are building can make a surprising difference and it can take years of experience to be able to predict the difference that even a difference of an inch of thickness in some layers can make. There is more about duplicating or approximating another mattress in post #9 here and some comments about designing and building your own mattress in post #15 here. It’s not generally something I suggest unless you have access to the same layers and materials and a very similar cover or have enough experience to be able to “translate” one design into another if there are any differences.
When you are buying individual layers and you end up making one wrong choice in terms of thickness or ILD then they are often not refundable (or there are costs involved in a refund) and if you replace it with a more suitable layer it can end up costing more than it would have if you had purchased the original mattress in the first place and had good exchange options available after a purchase that are a much lower cost.
Phoenix