Hi domiagnew,
You could use ILD if the type and blend of the Dunlop is the same and the ILD is measured in the same way and is accurately described but density is often a more reliable way to make firmness comparisons with Dunlop than ILD. It will depend on how the specific manufacturers you are comparing measure firmness and on their accuracy either way.
No … polyfoam is completely different from latex and with polyfoam (or memory foam) density is a “quality/durability spec” not a “comfort or firmness spec” as it is with latex. There is more about the different specifications that can make a difference in how soft or firm a foam material feels in post #4 here and more about the different ways that one mattress can match another one in post #9 here but there are thousands of different types of materials and components and many thousands of different types of mattresses that use each of them in different combinations and designs and in most cases (with a few exceptions) … a mattress that is available locally won’t have an exact or even a similar match that is available online (or vice versa). Local testing can give you some good reference points about the types of materials and mattresses that you tend to prefer in more general terms but it won’t usually be a reliable way to know how another mattress “feels” or performs in more specific terms unless there are only very small and fairly insignificant differences between them which may be difficult or sometimes impossible to find.
Trying to use “specs” or another mattress as your “target” (instead of a common set of criteria that you can use to assess every mattress you consider) can become overwhelmingly complex and in many cases can lead to information overload or “paralysis by analysis”. In most cases with more complex mattress designs or with materials that have too many variables involved that affect how they would feel to any particular person I would avoid trying to use specs or spec comparisons as a reliable way to choose or compare mattresses.
There is more about the different ways to choose the most suitable mattress (either locally or online) and how to identify and minimize the risks involved with each of them in post #2 here.
You may be overthinking this somewhat or relying on specs that may be too complex or that you may not have enough experience to make sense of or that don’t “translate” into your own experience and I would keep your choices and your methods of choosing a mattress more simple which can also be less time consuming and frustrating.
Phoenix