Hi erinaustin,
Your question may have a much more complex answer and take a great deal of knowledge and experience to fully understand (mattress design theory and interactions can be very complex) and normally I would just suggest to “trust” the suggestions of a knowledgeable and experienced manufacturer like SleepEz for the mattresses they make and sell but at the risk of leading you into a state of “paralysis by analysis” … I’ll provide a few insights that may help.
In general terms … most people would not really notice much difference in ILD in the range of about 3 - 4 ILD variations or less in the comfort layers although some people may be a bit more sensitive. Latex is also not manufactured to “exact” ILD’s and there may be a variance along the surface of the layer. In other words … the layers you tested which have an “exact” ILD may not be so exact. Talalay will have a narrower range when it is tested in different areas of the layer and is averaged out to a “rating” and will typically have an ILD range of =/- 2 ILD across the surface from it’s ILD rating. The range of Dunlop is usually greater than this and the firmness of Dunlop can also depend on which side is up and where in a thicker 6" core it was cut from (the bottom of a Dunlop layer is generally firmer to some degree than the top). For most people … these variations are not consequential because of the natural elasticity of latex and because it gets firmer as you compress it more so the variations in real life are not that significant or noticeable for most people.
The deeper in the mattress a layer is the less it will generally affect the “feel” of the mattress although it may still affect things like support/alignment which are not as big a part of how a mattress feels to most people but can have an effect on how they feel when they wake up. In general the top part of a mattress will contribute more to how it feels for most people than the bottom part but the heavier you are the more you will tend to notice deeper layers in the mattress.
Different people have different body types and different sleeping positions and sensitivities to different types of “softness” (see post #15 here) and this will also affect how they perceive any differences in layering and which layers of the mattress they will tend to feel the most. As an example … people who sink into a mattress more (heavier or side sleepers) will notice more of the similarities in a thicker part of the top layers while people who sink in less (lighter or back or stomach sleepers) will notice more of the similarities in a thinner part of the top layers in terms of pressure relief.
Finally … the thickness of a layer has as big an effect on how it feels as the softness does. If you have thinner layers then you will “go through” it more and feel more of the firmness/softness of the layers below it. This means that different designs need to be “translated” into each other to see how them may compare.
This seems to “point to” a 2" top layer in a similar or softer ILD range over a slightly softer middle layer (such as the 9000) which would be “in between” or a 3" top layer that was slightly firmer in combination with a middle layer that is slightly softer which would also be “in between” (like the 1000).
In a 3 layer mattress … the middle layer is a “transition” layer and will partly affect the pressure relief of the upper layer and will partly affect the support of the deeper layer. If it is softer than the base layer you tested then it will create a slightly softer surface (depending on the weight of the person and how deeply they sink into the mattress) and slightly soften the support compared to the firmer layer you tested. The deepest layer would have a similar but lesser effect and if it is firmer than the support layer you tested then it would “help” the middle layer to have slightly firmer support. As you can see there are many interacting effects all happening at the same time.
This can have so many variables that it is impossible to predict for someone specifically even with a great deal of personal experience but I would put more emphasis on the top half of the mattress being “similar” (which is the part that creates most of the pressure relief and “feel” of the mattress) and make the bottom half progressively firmer to provide the best possible support (which you will be thankful for in the morning). Good support is especially important for good alignment for stomach sleepers because stomach sleeping is a more risky sleeping position and can easily lead to alignment and back issues if the top layer is too soft or thick or the support layers are too soft.
To go into a little more specific detail … (and assuming that the soft in the SleepEz is 23 ILD and the medium is 31 ILD) and that
Mattress #1 is the one with 2" of 26 ILD over 6" of 35 ILD
Mattress #2 is the one with 4" of 26 ILD over 6" of 35 ILD
Mattress #3 is the SleepEz 9000 with 2" of 23 ILD over 3" of 31 ILD over 3" of 40 ILD.
Mattress #4 is the SleepEz 10000 with 3" of 31 ILD over 3" of 31 ILD over 3" of 40 ILD.
Then AS AN EXAMPLE ONLY to illustrate the idea (don’t take this as a prediction of what you will actually feel in other words) If the top 40% was the most important part of the feel and “pressure relief softness” of the mattress for you …
Mattress #1 would “average out” to 26 x 2 + 35 x 1.2 / 3.2 = 29.4 ILD in the top 40%.
Mattress #2 would “average out” to 26 x 4 / 4 = 26 ILD.
Mattress #3 (the 9000) would average out to2 x 23 + 1.2 x 31 / 3.2 = 26 ILD.
Mattress #4 (the 10000) would “average out” to 31 x 3 + 31 x 1.6 / 3.6 = 31 ILD.
I am using “averages” just to illustrate the point because layers closer to the surface will have more effect on pressure relief than deeper layers.
If the bottom 75% was the most important part of the support/alignment softness of the mattress … then once again AS AN EXAMPLE ONLY (and this time I’ll just go right to the ILD)
Mattress #1 would be 35.
Mattress #2 would be 33.2.
Mattress #3 (the 9000) would be 35.5.
Mattress #4 (the 10000) would be 35.
Again … averaging out can be a little misleading because in the support layers as well the layers closer to the body will have a greater effect than the deepest layers.
As you can see … these would all have different combinations of “pressure relief softness” and “support softness”. In this hypothetical example … both of the SleepEz versions would be the same or more “supportive” than either of the other two. The SleepEz 9000 would be roughly the same as #2 in terms of pressure relief and the SleepEz 10000 would be roughly the same (a bit firmer) than #1 in terms of pressure relief. I should stress once again though that this is to illustrate the different types of softness not to predict what you will feel on any of these mattresses.
Stomach sleepers tend to do better with firmer overall support (so their heavier pelvis doesnt sink in overall as much and lead to sleeping in a swayback position), back sleepers need a little less, and side sleepers need the least of all (support here means overall support because there is no specific line where a layer becomes either a comfort or support layer).
As you can see, Shawn has suggested support layers based on your testing and his experience and comfort layers that are roughly at either end of the range of your testing experience with good support in both instances.
As I mentioned at the beginning I would normally just suggest going by Shawn’s suggestions because they are generally in the best interests of his customers and based on his experience, knowledge and intuition within the limitations of the layering they have available. They would normally be the best place to start (and hopefully end).
I wanted to give you a bit of insight though into some of the “theory” behind various combinations at the risk of causing “paralysis by analysis” caused by analyzing details that can take a long time and a great deal of experience to fully understand when the layers are not the same and need to be “translated”.
Hope this helps more than it “hurts” 
Phoenix