Hi Apeiron,
Welcome to the Mattress Forum! 
I appreciate the level of detail you provided in your post, but I’ll attempt to keep my response more succinct and general, as I fear you’ve gone a bit “down the rabbit hole” of analyzing your choice and are straying from the most important thing - your own sleeping and personal testing experiences - and I don’t want to feed into any potential over-analysis of product.
I’ll address a few simple “bookkeeping” things first, then get on to your main concerns.
You’re correct that the foundation and frame play an important part in your mattress comfort life and support/alignment, so I’m happy that you’ve addresses those things.
I’m sorry that your Spindle mattress didn’t work out for you, but I’m glad that Kim and Neal were very helpful. As you’re aware, Spindle is a member here of the site, and I do think highly of Neal and his expertise.
While Dunlop “firms up faster” than Talalay, there certainly was enough material in your tested configurations to allow you to sink in, but there is much more involved in achieving the comfort you desire than just the compression modulus, including the density/ILD of the Dunlop layers used, synthetic vs. natural, fillers, layer ordering, overall layers and their thickness, and even the covering, just to name a few things. It certainly may be that you have an affinity for the feel of Talalay over Dunlop, but I would avoid the temptation to try and get “too technical” for your reasons, and instead focus upon your impressions of comfort when testing out the product.
No, perhaps you mean 90 kg/m3 for density, but more accurately firmer Mountain Top latex used by Spindle is in the 5.5 lb/ft3 density range.
This is probably correct. Too soft/too thick of an upper comfort layer can contribute to poor alignment and also allow to “sink in” too much, allowing for a lateral curve which can be felt along the ribs by some people. Memory foam itself is not a supportive material, and you were probably “feeling through” the memory foam layer to the firm latex beneath it.
Regarding the Savvy Rest mattresses you tested, they use Talalay from Radium and it is 100% Natural. Their soft is in the 15-20 (N5) ILD range, their medium is in the 30-37 ILD (N7) range, and their firm is in the 37-44 ILD (N8) range. The Dunlop in the Savvy Rest mattresses is from Coco-Latex and is 100% Natural. The soft is in the ILD range of 22-30, the medium is in the ILD range 31-39, the firm is in the ILD range of 40+.
As a reference, Talalay in the western world will be sourced from either Radium or Talalay Global (TG). Both companies supply different formulations, some which can be proprietary for larger clients. As an example, Radium has their Embrace, Intuition, Natural Superior and FRX lines, while Talalay Global had additives like phase change materials, graphite and copper they will put in some specialty products.
They are both available in either 100% natural or in a 30/70 blend (NR/SBR) and in practical terms I would treat them as being comparable in terms of durability.
TG blended Talalay comes in target ILD’s of 14, 19, 24, 28, 32, 36, 40, & 44. You can see the approximate density of each of these in post #2 here.
Radium blended Talalay comes in target ILD’s (rounded to the closest whole number) of 11, 15, 19, 22, 25, 29, 34, & 40
Talalay Global uses a filler in their latex formula while Radium doesn’t
Talalay Global 100% natural Talalay comes in N1 (14 - 19), N2 (20 - 24), N3 (25 - 29), N4 (30 - 34), & N5 (35 - 42) with nominal midpoints of about 17, 22, 27, 32, 38.
Radium 100% natural has ILD targets (rounded to the closest whole number) of 9, 13, 18, 23, 28, 33, 39, and 44
You can see a chart for Radium’sILDrangehere.
Most of the manufacturers that I’ve talked that work extensively with both say that they have a comparable “feel” although of course this is subjective, and they all tend to agree that both brands are superior foams. The difference between them would be relatively small and would probably reflect the different ILDs, amount of filler, and any other differences in compounding formulas and pincore patterns they have.
In practical terms … while there are slight differences between them … I would treat them as equals and the fact that both are Talalay (either blended or natural) would outweigh for me the source of the latex. Radium tends to dominate the Talalay latex market in Europe and Talalay Global tends to dominate the North American market and both of them also do business in each other’s primary markets. They are both well respected and quality products and I would focus more on issues such as which type of Talalay I wanted, the ILDs of the layers I wanted, and the overall value of the mattress rather than on one brand over another.
As for the Savvy Rest configurations you tested, the St, Md, Fd was a bit too soft on top for you.
The Mt, Md, Fd was very, very good. Just soft enough for your ribs, but firm enough for alignment of the hips and shoulders.
The key with any configuration is making sure that you find something with deep support and alignment, then “just enough” comfort material on top to provide adequate relief. Overall, people tend to skew too far to the plush end of the spectrum (something Spindle alluded to earlier).
If you desire something with just a bit more surface comfort, you could look for Radium’s 100% Natural in N6, or a Talalay in the mid-20s range. But if you’re considering making your own DIY mattress, realize that all of the componentry works together, so any change in one layer, even the covering, will make a change in the overall feel of the product.
I’m not going to address your other questions about why you don’t see as many N2 or N3 layers being used by people, or who might find them appropriate, as much of this has nothing to do with what might work for you personally and I think my answers would only serve to drive you further “down that rabbit hole”. And attempting to get too technical and predict what might work for you when you aren’t technically experienced in mattress design, specifically latex mattress design (and this would be very few people nationally - it’s nothing against you personally), becomes a bit of a futile exercise, as nothing can replace your own personal testing on these different configurations, and I would put the most guidance in what you’ve tested in person.
Overall, I think you are on a good tract with your thoughts on perhaps trying out something in the uppermost layer between the two Talalay layers you tried, but if you had to choose one of the two you have already tested, I would personally lean toward the Medium Talalay over the Soft Talalay, based upon your previous comments.
If you have more specific questions or comments I’ll do my best to answer those, but I’m purposely keeping my thoughts to a more general range for right now to focus specifically on you and your needs. I hope that helps.
Phoenix