Hi amnj,
Welcome to the Mattress Forum! 
You’re welcome.
Being a good motion-isolating material, memory foam is generally the most difficult item in which to reposition - you are correct.
There would be little practical difference in durability between 2 – 3" support latex layers and 1 – 6" support latex layer is the latex was the same across the board. The real benefit would come from the ability to more easily customize the configuration using 2 – 3" layers.
There is a bit of a difference in the feel between Dunlop and Talalay latex, with more difference being felt the closer to the top of the mattress that the foam is located.
The latex resilience is a measure of how high a steel ball dropped on a material will bounce expressed as a percentage of it’s original height. It is somewhat opposite to hysteresis which is how much energy it absorbs (called hysteresis). Both Talalay and Dunlop are highly resilient but Talalay has greater resilience (a ball will bounce higher) and springs back more powerfully while Dunlop has less hysteresis (it absorbs less energy overall but it doesn’t decompress as strongly). The biggest difference between them is what is called compression modulus. This is a measure of how quickly a foam gets firmer as it is compressed more. ILD is generally (but not always) a measure of how much weight it takes to compress a foam by 25% of it’s thickness. At compression depths less than this Dunlop will be softer (compress more with the same weight) while at compression depths more than this Dunlop will be firmer (compress less with the same weight). They have a different response curve in other words and since Dunlop gets firmer faster than Talalay … even though the different response curves may cross at the 25% compression level … beyond this Dunlop will not compress as much as Talalay which is why it is generally considered more supportive or “firmer”. Another way to say this is that Dunlop starts off softer and then ends up (at compression levels more than 25%) firmer than Talalay.
This is also part of the reason that Talalay is considered to be more “springy” or “lively” than Dunlop because you will generally sink in deeper which means there is more up and down “movement” or “ride” with Talalay and it springs back more strongly and quickly. This creates a different “feel” between the two materials and is also why Talalay is often considered to be more pressure relieving than Dunlop because it allows for a deeper cradle in the same ILD … while Dunlop is considered to be more supportive because it doesn’t compress as deeply with greater weight … all else (including ILD) being equal of course. Some people will prefer the feel of one over another and because each has different firmness levels available … one is not “necessarily” better than another in either pressure relief or support layers if the right layering or ILD is chosen for each … but you may need to choose a softer ILD with Dunlop than with Talalay to get similar pressure relief in the comfort layers or a firmer Talalay vs Dunlop to get similar levels of support in the support layers. A “one step” difference in ILD which would be about 4 - 5 ILD would be “in the range” for most people where they felt similar.
Regarding your choices between natural or blended latex, in practical terms their differences are relatively small in the cured foam and many people would not feel significant differences between the two in side by side testing. Natural would be slightly more “springy or buoyant” and more “supportive” while synthetic would generally feel a little softer and be slightly more pressure relieving (would allow for slightly more sinking in and cradling). The thicker the layers that are being compared the more it may be possible to feel a difference (for example someone may not feel the difference between two 3" layers of each material but they may feel a difference between 9" of each material).
In the higher ILD’s which use more rubber and less air … any differences in durability between them would be negligible. In the lower ILD’s which use less rubber … these differences may be more noticeable. Talalay Global acknowledges the differences between their natural and blended Talalay (their blended is a bit more durable) while Radium has told me that they use a special curing past for their 100% natural in the lower ILD’s which should negate any differences in durability.
So…the bottom line is Talalay made from natural rubber is more expensive, comes in a less specific range of ILD’s, is heavier, slightly more elastic and springy, has a higher compression modulus and more supportive, may get softer and develop impressions slightly faster in lower ILD’s, and is more natural. The primary reason that it was introduced (in 2005) is to cater to the “natural” market who are willing to pay more for a product which has a more natural source even though it may not be as durable.
Those who value a more natural product for personal reasons and are willing to pay for it (in price and possibly lower durability), or who can feel a difference and prefer the feel that comes from the greater elasticity and compression modulus of the natural would likely choose the 100% natural Talalay.
Those who value the increased consistency, greater choice of softness/firmness levels, or lower cost of the blend or who can feel a difference in pressure relief, would likely choose the blend.
All are great choices and in the end the choice between them is part of each person’s individual “value equation”
You can see some of my general comments above regarding the general differences in feel between Dunlop and Talalay latex. The layers closest to your body will of course have the greatest impact your overall comfort, so avoiding too much latex in the lower ILDs (softer) will certainly be assistive. Dunlop will also tend to feel a bit “firmer” than the corresponding ILD of Talalay, but if you’re sticking to a thinner layer of soft Talalay on top and if there is a correspondingly firmer layer of Talalay or Dunlop latex below you shouldn’t have too much of a “quicksand” feeling.
Choosing a mattress and the individual layers for you isn’t something that I’m able to do (I can only offer information on “how” to choose), as there are entirely too many individual variables and preferences to take into account and forming an algorithm to predict what might work best for you.
If you’re considering ordering something online, then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer (like those forum members listed here) that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (“quicksand feeling”, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done on the previous mattresses you mentioned or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
If you’re visiting other retail stores, I would use the specs (if they are available) of the mattresses that you have tested and confirmed are a good match for you (if you did find one) in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible, as you’ve already had some experience testing a few latex mattresses in person.
I know this may be a bit of a longer reply than you expected, but hopefully the information I provided will be useful as you search out a latex mattress that isn’t “too soft and quicksandy” on top.
Phoenix