Hi kakhea.
Welcome to our Mattress Forum! 
I’m happy that the information here is helpful and is making things a bit clearer for you.
Thanks for summarizing at the end of your posting the few things to keep in mind …sometimes I have to dig across many postings to understand the larger picture for any particular poster.
You are correct that all 3 Ikea mattresses you’ve tested are rated as medium firm , but generally speaking, there are no “standard” definitions or consensus of opinions for firmness ratings even across products from the same manufacturer where accuracy is not a priority. As all layers work together, how much you’ll sink an what you feel on a specific mattress depends on your body specifics and also on the overall layer combination, including the encasement and the bedding being used and also as the thickness and softness are interrelated and this may also may alter the firmness rating as it applies to you.
The issue of firmness ratings is even more complicated as different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here ) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
Martrand Latex 7" uses 5" of synthetic Dunlop latex (A forum search on Myrbacka and on Matrand (you can just click the links) will bring up more information about them (and some pictures as well)
While the Dunlop latex would be a better quality material, and the polyfoam would be “on the edge” of being better quality, I have received feedback in the past on the inconsistency and poor quality of assembly of many Ikea mattresses, so it is possible that you run into this when you tested this item. I would not rush into any purchase just because it is convenient or because you wish “to get it over with”. The way you sleep will affect the way you live and it is worth the time investment to get things right.
The most important part of the “value” of a mattress purchase is how well it matches your specific needs and preferences in terms of PPP and this can be very different for each person. Regardless of the cost or quality of a mattress … if a mattress isn’t a good match for you and you don’t sleep well on it then it would have little value to you regardless of how well any other person or group of people may sleep on it. When you are looking at a local mattress then careful and objective testing is the most reliable way to assess this.
Outside of the suitability of a mattress … the quality and durability of the materials are the next most important part of the value of a mattress purchase. If a mattress is “perfect” at first but uses lower quality materials that can soften, break down, or compress too quickly relative to the price you paid and you lose the comfort and support that was the reason you bought it in the first place then it would also have little value to you. Don’t forget that the loss of comfort and/or support is the main reason that you will need to replace a mattress and this isn’t covered by warranties (which only cover manufacturing defects). In the case of latex I would want to confirm the type and blend of the latex (unlike polyfoam or memory foam where the density is the most important “quality” spec).
The answers to your questions about ILD may be a little bit more complex and more than you would want to know at this time. Suffice to know that that ILD in latex has variance across the surface (usually measured with a 6" thick layer) differs from manufacturer to manufacturer (may be using different testing protocols) The accuracy of the rating depends on the source and their knowledge of latex foam and on whether the ILD is a real measurement or just a guess or “mistranslation” from other methods. Dunlop latex that is accurately rated as being in a certain range (it’s never a single number in reality) will also feel firmer to most people than Talalay of the same ILD range unless you only sink into the layer exactly 25%.
If you are testing a mattress locally then none of this really matters because what you feel when you test a mattress will be more important than any ILD or comfort specs although it may be more important in an online purchase if you know the specific and accurate ILD of all the layers in a mattress you have tested locally (as well as all the other information and specs that will play just as big a role in how a mattress feels as ILD) and you are looking to make an online choice that is somewhat comparable to what you have tested locally.
As far as adding more layers …In the large majority of cases … 8" - 9" of latex is easily enough to include the combination softer layers (or sometimes sections) for pressure relief and firmer layers for support that most people of average or even higher weights would need. In some cases … lighter weights or people that sleep in “flatter” sleeping positions, have slimmer less curvy body types, or who prefer a firmer mattress will do well with even 6" even though there is less “room” to design in different layers in the mattress. Thicker mattresses can also use firmer materials because thickness and softness are very related and work together. These are all good questions to ask the manufacturer or retailer of a mattress so they can discuss the differences between the different options they have available in terms that are more specific to the layers they use or options they provide.
Unfortunately there is no “magic bullet” when it comes to your own needs and preferences … “suitability” is the most important part of the “value” of a mattress purchase is and how well it matches you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) which “translates” into how well you will sleep on the mattress. Only you can feel what you feel on a mattress or decide which one you would most likely prefer and that you believe is likely to be the best match for you based on your preferences and testing on different materials and types of mattresses (using the testing guidelines in the tutorial post) or in the case of an online purchase your more detailed conversations with an online manufacturer or retailer so they can help “talk you through” the options that may be best for you (see post #2 here) There is also more about the different ways to choose the most suitable mattress that can help you identify and minimize the risks involved with each of them in post #2 here . There would be little value in a mattress purchase where you don’t sleep well on the mattress regardless of the quality of the materials or the price of the mattress.
Outside your personal testing one best way to get a more accurate idea is to deal with better manufacturers or sources who will usually list their latex more accurately as they are much more familiar with their own mattress designs and materials than anyone else and they can use the information you provide them about your body type and sleeping positions, your preferences, your history on different mattresses, and the results of your local testing to make suggestions based on the “averages” of other customers that may be similar to you. The more accurate and detailed the information you provide them the better you will help them to help you make the best possible choices out of the options they have available. Of course the options you have available with each retailer or manufacturer (or with a particular mattress) and your ability to exchange layers or the mattress itself or use other forms of fine-tuning after your purchase or the return policy may also be an important part of your personal value equation or to offset the risk that can go with any online purchase. Both Flexus Comfort and Latex Mattress Factory along with other members of this site would qualify as “better” or more accurate sources. They are both our Trusted Members here which means that I think very highly of them and that I believe that they compete well with best in the industry in terms of their quality, value, service, knowledge, and transparency.
So hopefully this wasn’t too complex even though it’s never as simple as I and most others would want it to be.
Phoenix