Hi Dag,
Unfortunately there are so many variables of body type, sleeping style, physiological condition, and preferences, that there is no formula that can “predict” which type of layering you will do best with except in very general terms or “theoretical” terms. This means that there are really only 3 ways to know which layering you would tend to do best with.
So while I can’t tell you “what” to choose (that should always be based on more detailed conversations between you and the manufacturer or retailer you are working with who have much more detailed knowledge of the materials they use and the options they have available than I do) … I may be able to help you with “how” to choose.
The first of these is carefully testing a mattress in person. I know you mentioned that you didn’t think there were any Dunlop mattresses available close to you that you could test in person but there may be some options that you haven’t found in your area and if you let me know your city or zip I’d be happy to let you know of any I’m aware of.
The second is to talk in more detail with a manufacturer or retailer you are considering who can help you choose a mattress layering that “on average” matches others in their customer base that seem similar to you. While you may not fit the “averages” that they use … at least this would put the odds of success much more in your favor.
Finally you can order a mattress where you can re-arrange or exchange layers so that if your own personal experience on the mattress doesn’t “match” what you are hoping for then you can either re-arrange layers or exchange layers for others that are firmer or softer.
If the first option is not available where you can test mattresses in person either as a purchase or as a guideline for an online purchase … then I would strongly suggest a purchase which are a combination of #2 and #3 where you can start off with a mattress where the odds are better that it will match what you need and prefer but where you can either re-arrange or exchange layers to fine tune it based on your actual experience.
A list of some of the manufacturing members here that sell latex (including natural Dunlop) where this is available is in post #21 here.
I also think that you may need more thickness in a mattress both to provide layering options and to accommodate your husband’s higher weight. You can read more about the potential benefits of a thicker mattress in post #14 here.
Layer thickness and layer softness work hand in hand to create the performance and feel of a mattress and I think that the combination of 45 ILD Dunlop and only having 6" of latex in your mattress would create a mattress that is too firm for the vast majority of side sleepers.
Our needs and preferences and the condition of our bodies also change over the decades and what once was great for us to sleep on may not work nearly as well a few decades later.
You may also have different needs and preferences because of your different body types so each of you may decide on different layering combinations. I would also keep in mind that foam gets firmer as you compress it more and latex gets firmer faster than other types of foam. This is called the compression modulus of a foam (also support factor, sag factor and other names as well) and Dunlop has a higher compression modulus than Talalay. This doesn’t mean it is “better” … only that it gets firmer faster and either one may be beneficial in certain designs or particular body types. Some people may need to sink in more and others may need to sink in less but in both cases it’s usually a good idea to use softer foam on top than 45 ILD because latex that starts off softer will get firmer as you sink in deeper and is very supportive compared to other foams. I would guess at a minimum that medium would be a better choice but again … thicker may work better yet because it can be more adaptive to different weights and has a greater “range” of compression than thinner mattresses.
Since you already have two “hard” layers … then it would make sense to use these as the base layer for both of your mattresses and then build on this with softer layers to “complete” both mattresses with softer layers.
In the large majority of cases this will be fine. While I tend to prefer a slatted base when other choices make it possible, it would usually take a combination of factors to create the conditions where mold and mildew with a latex mattress becomes an issue. You can read more in post #10 here. I also use a solid surface adjustable bed because we wanted the adjustable and any additional risk was worth it to us.
The goal is always spinal alignment and both mattress that are too hard and too soft can both cause issues in alignment for many reasons including that they can “tilt” the pelvis outside of it’s neutral range which in turn changes the curvature of the spine. Pelvic tilt in either direction can cause back issues. Excess pressure in some areas (like the hips) or lack of flexibility in certain joints can also cause the spine to twist which can also cause back issues. The goal is to have the primary support that “stops” the pelvis from sinking in or “tilting” too much in either direction and at the same time to have enough softness in the top layers to “allow” the bony prominences to sink in enough and provide the secondary support that fills in the recessed areas of the spinal curves and helps maintain their natural alignment.
I should also mention that Dunlop is not typically more durable than Talalay and in practical terms I would treat them as “equals”.
Hopefully this can give you some good ideas and some very knowledgeable people to talk with on the phone so they can add their suggestions and information to mine and both of you can start sleeping better very soon 
Phoenix