Hi Theodore,
You’re very welcome … and I couldn’t agree with you more that a mattress can be an expensive mistake which of course means tilting the odds more in your favor can be an important part of the process … which of course is exactly what you are doing 
As you know … you are a good candidate for split layering because your needs and preferences may be slightly different. It’s not unusual at all for stomach sleeping to cause back problems because the best layering for side and back sleeping often needs a comfort layer which is too soft and thick for stomach sleeping which needs a thinner and firmer comfort layer to prevent sleeping in a swayback (hyperextended) alignment. Stomach sleeping is by far the riskiest and most problematic of all the sleeping positions. This can be particularly true when it is combined with side sleeping. The best solution is usually to choose slightly firmer than “typical” for side sleeping rather than slightly softer so that you have “just barely enough” pressure relief on your side.
As you probably know, SleepEz has more options at better prices than Savvy Rest. While Savvy Rest doesn’t list their exact ILD’s … their soft Talalay would be roughly comparable to SleepEz’s soft all natural talalay and their Dunlop ratings would also be roughly comparable to SleepEz firmness ratings (Savvy Rest uses Coco latex for their dunlop and SleepEz uses Latex Green but both are equivalent quality). ILD’s are not exact … especially in Dunlop … but your best option would likely be to choose similar layering using similar materials if you were ordering from sleepEz.
Blended Talalay will be a little different than all natural but as you mentioned in softer ILD’s is likely to be more durable and while it may also be a little softer in a comparable ILD … the difference wouldn’t be enough for most people to notice if they were comparing lying on 3" of soft all natural talalay and 3" of blended talalay if the ILD’s were the same. Dunlop is also firmer than Talalay of the same ILD because it gets firmer faster with deeper compression so the soft Talalay would be noticeably softer than soft Dunlop.
Because your testing was not as “specific” as it could have been … you may be better off either with a little more “specific” testing at savvy rest to narrow down the best choices or to choose a fairly wide range of layering to do some sleep testing at home with the SleepEz and sort of “build in” the possibliity of a layer exchange to do any fine tuning that sleeping on the mattress for a few weeks may indicate is necessary.
If I was in your shoes … I would tend towards Soft talalay/medium Dunlop/ Firm Dunlop for you. This would let you exchange the 2 base layers to test if a firmer layer in the middle would work better for you (S/F/M) and would give you a clear picture if a layer exchange was needed after sleeping on it for a few weeks. This would likely be the layering with the best odds for you (unless of course you chose talalay in all three layers which would be slightly softer overall). The firmer the support layers are though, the more likely it would be to help with your stomach sleeping (this would depend on how much time you spent on your stomach as well) because a 3" soft comfort layer is usually good for side sleeping but not as great for stomach sleeping.
Using 3 different firmnesses would also help with fine tuning more than having two identical lower layers. For most people … the top layer has the most effect and the bottom layer has the least effect on the overall feel but there are many who would notice a difference with a firmer bottom layer and of course even if they feel the same (if the difference is below someone’s perception threshhold) … the support would be a little better with a firmer bottom layer. While I would tend to avoid soft Dunlop as your middle layer … you would also have the option of choosing medium Talalay in the middle which would be a little softer than the medium Dunlop but firmer than the Soft Dunlop.
Your husband on the other hand is sort of in a middle ground between the same layering but may do better with slightly firmer options. Because of his greater weight, he may do better with medium talalay/ firm Dunlop / Extra firm Dunlop. Medium Talalay will be softer than Medium Dunlop (which he didn’t seem to like). While this may be a little firm for him … it would also give both of you the chance to switch top layers to see if either of you preferred the medium talalay over the soft talalay and again you would have the time to “zone in” your best choices over a few weeks sleeping experience. With a 3" comfort layer … slightly firmer would also be a little better for back sleeping but again he would have the chance to test the soft/firm/X firm combination (or any others that your combined layering made possible). Because he is quite tall … his extra weight is distributed over a larger frame so he may end up preferring something similar to you (or perhaps slightly firmer in the base layers).
Latex (either blended talalay or natural Dunlop) is the most durable of all foams and while anything will soften or even “sag” after a long enough period of time (softer faster than firmer because it has more mechanical compression) … it will take a much longer time than any other foams. The advantage of the type of mattress you are choosing is that if over time the top layer does soften too much (and this would take a long time) … then it would just be a matter of replacing a single layer rather than the whole mattress because the top layer is the one that is the most “stressed” in a mattress.
I hope this didn’t sound too complex but I would tend to try one more round of more specific testing" with Savvy Rest and then make your choices based on your more “accurate” PPP (pressure relief, posture and alignment, and preferences) experiences. Ultimately this will be a more accurate indicator of long term satisfaction than just "comfort.
Failing that (more specific testing) I would probably go with S Talalay/M Dunlop/F Dunlop for you and M Talalay/ F Dunlop/X firm Dunlop for him because this gives you the greatest range of layering to zone in your final choices unless your testing pointed in a different direction.
Hope this helps
Phoenix
I could noticeably feel the firmer side pushing my hips up when I was on my back and I thought it was too much push. On the softer side - it was so soft that I thought although it felt dreamy for the minute I laid there I probably shouldn’t go that soft either.