Hi ehuesman,
That’s unfortunate. They certainly aren’t doing themselves any favors by having salespeople with so little knowledge. Fortunately this is more the exception with most local manufacturers.
Unfortunately … “organic” can sometimes be a “marketing code” for expensive. I tend to share your sentiments overall (although organic is more important to some than to others) and you can see some of my thoughts about “organic” it as it pertains to latex in post #6 here.
Post #4 here has more about trying to duplicate another mattress and the reasons why it can be very difficult to do more than “approximate” it. It’s usually best to “rate” each mattress you test against a common set of criteria rather than using another mattress as your 'target".
ILD is only one of the measurements used to rate the softness of a mattress and it is usually best used to compare the same materials but having said that all natural Talalay will feel very similar to most people in a 3" layer to blended Talalay although some who are more sensitive may notice some difference between them because natural Talalay is a denser material which will get firmer a little faster than blended Talalay in the same ILD and an ILD measurement is only completely accurate if you happen to sink into a layer by exactly 25% (how ILD is usually measured).
I would keep in mind that this would be more true in separate comfort layers that are closer to the surface than it would be in deeper layers where the difference may be outside of most people’s perceptions.
Two layers with a different ILD will have a different compression modulus (a measurement of how quickly a layer or layers get firmer with deeper compression) than a single layer with a single ILD and compression modulus can have just as big an effect as ILD on how a material feels in real life. Other factors such as point elasticity and different compression curves between different materials can also play a role in how soft or firm they feel for different people. Having said that … the layers that are closer to you will be a more obvious part of how a mattress feels for most people so if a middle layer was the same as the upper half of a single layer it would be relatively close for most people (depending on weight and individual sensitivity to different “types” of softness) so if the medium firm layer was the same type of latex and had the same ILD or perhaps a little firmer than the medium middle layer over extra firm then it would probably be fairly close … but again not for all people. The closest would be to have two thinner layers which were the same ILD as the single layer.
The upper pressure relieving layers usually have a bigger effect on the apparent “comfort” of a mattress for most people so if the top layers were the same and were in the range of 3" or so then for many people it would be a reasonable approximation. This would also depend on how heavy they were which will change how much of an effect the middle and even lower layers have on how a mattress feels and performs. Heavier people (or side sleepers) sink into a mattress more than lighter people or other sleeping positions so the lighter you are the more likely that they would “approximate” each other in terms of “feel” although they may still be quite different in terms of performance in ways that aren’t as readily apparent (such as alignment and how they respond to changes in position). You can read a little more about the difference between a latex/polyfoam hybrid and an all latex mattress in post #2 here.
Post #15 here has more of my thoughts about this and while a component mattress can be a good idea if you are working with a manufacturer that has the knowledge and experience to give you some guidance and offers layer exchanges and materials and components that are known to perform well together … I would discourage trying to do it on your own in most cases because mistakes can be costly both financially and in terms of sleeping quality. I have seen too many people end up with mattresses that just don’t work the way they want them to when they try to become their own mattress designer or when they try to make changes without really knowing what change will have what effect they are looking for and without really having the knowledge and experience to build their own mattress successfully. It may also not be significantly less compared to some of the value in component mattresses that are available.
While I understand that cost is an important part of value and sometimes doing everything yourself can also be worth it for some people just for the challenge, “fun” and learning experience alone … years down the road you will remember much more about how well you slept on a mattress and on how suitable it was for you than you will about relatively small differences in price IMO.
Phoenix