Hi Barnaby,
I can see that you’ve done some homework and you are certainly going in some great directions IMO. Thank you too for the donation 
On to your questions …
This will depend to some degree on the type of memory foam you use and how deeply you sink into the mattress (which will also be dependent on the other layers besides just the memory foam), the type of fabric used in the cover (natural fibers are the most breathable and there are other “co0ling” covers as well) and on the sheets you use (good quality cotton, bamboo, or linen would make good choices). If it is a more breathable memory foam then most people probably wouldn’t require the low density foam over it. There are also some HD memory foams (7 - 8 lbs) that are more breathable than others and a layer of this over 5 lb memory foam may also be worth considering.
The challenge with putting layers over the memory foam (either low density foam or wool) is that it will also affect the feel of the memory foam below it as well and slow down the rate that heat reaches it which slows down it’s softening response. This can be a good option for “feel” because it gives you a more resilient surface but it also means that it is particularly important to use layers that are not too thick so that you don’t end up sinking in too far over the course of the night even though it may feel good at first. A 2-3" layer is a good choice for thickness IMO but by itself it probably won’t be enough for your weights and side sleeping without some “help” from one or a combination of a quilting layer (used for softness instead of ventilation in this case), a layer of latex or HD memory foam over it, a transition layer underneath the memory foam (again latex could make a good choice), or a support system that has some “softness” in the initial response of the spring before it firms up.
There are probably as many ways to describe the “feel” of latex as there are people. Part of how it feels will also be dictated by the layers above and below it. In general though … I personally would tend to call it “'springy” or “lively” compared to other foams which really is a metaphor for its resilience (the amount of energy it returns rather than absorbs), it’s compression modulus (the rate that it gets firmer), and it’s more instant response. It is closer to polyfoam than to memory foam because they are both instant response and there are even some types of polyfoam that have specs that are quite close to latex. I would definitely recommend lying on it though because it’s really your own “description” that will be the most important. Dunlop will be less “springy or lively” than Talalay.
This would be closer to my personal preference if I was to combine the two materials. It could be layered in whatever order you chose although I tend to prefer the latex on top because I prefer a more resilient surface. It’s interesting that both the iComfort Wellness refined and the Simmons Black Kelyn have latex in the mix (2" and 1.5" respectively) although they both include other materials besides just the memory foam and latex as well.
You may be more similar than you think but my guess would be that any split difference would not be large. Generally this is evened out with the quilting in the cover or a layer of “unsplit” foam on top of the split layer. For the majority of people a “1 step” split is evened out enough with the cover quilting and is not an issue but if the split has a wider differential between then … then you may want to consider a consistent foam layer over top. The necessity of a split would depend on how different you were in your needs and preferences in “real life”. It may be interesting to see how each of you do on the Tempurpedic HD lineup (Rhapsody, Allure, GrandBed) just to see if there is a mattress that you agree on. This will give you a chance to test the softer HD foam (it’s higher density but softer than their “regular” foam) over the firmer 5.3 lb memory foam as a reference point. They are a good reference point because most memory foam manufacturers have a rough idea (and sometimes a more specific idea) of how their mattresses compare to the Tempurpedic line.
Zoning can be tricky but can also be very helpful. Vertical zoning is really just a side effect of certain mattress layerings where a top and middle layer may be the effective comfort layer for a heavier person with the deeper layers being the effective support layer while the top layer is the effective comfort layer for the ligher person while the middle and lower layers are the support layers. In other words … the different layers perform different functions for different people. This is usually more effective when you have say a 120 lb person in combination with say a 250 lb person.
Horizontal zoning can also be effective in certain circumstances to help people where the hip pelvis area sink in too far relative to the other areas of the body. With materials that have a higher compression modulus (get firmer faster) … it is less necessary because it already does such a good job of creating alignment because of its ability to be both soft and supportive but even here it can help. By using firmer material in the middle under the heaviest area of the body … it allows for the use of softer material under the lighter shoulders which can allow them to sink in more. In general … when a mattress is zoned … I tend to prefer either 2 or 3 zones because more than this can present some challenge of “fitting” the zones and is also on the overkill side. This doesn’t mean they are “bad” (although they could be for some people) … only that they are unnecessary. Some types of zoning are more “reverse zoned” in that they “allow” the heavier areas to sink in more rather than “stopping” the heavier areas. These are particularly tricky and could cause issues for some people. There’s more about zoning here. In general I would tend to tick with 2 or 3 zones if I went in that direction.
I think that latex itself is very breathable and the convolution is used more for softness than breathability. While each manufacturer has their own design theory … the “standardized” 2" convoluted layer on the FloBed is less flexible than having options for what you want to use in the top layer of your mattress. Convoluting has a certain type of response (it is a way to increase compression modulus further depending on the shape and depth of the convolute) and can be beneficial but it also limits you to custom options below the top 2" layer. I don’t think “everyone” does best with a 2" convoluted layer of latex but having said that they also have the option of using a different layer instead. The biggest factor in heat is the layers closest to the surface so if you have memory foam over the latex … then the breathability of the memory foam is a much more important factor than the type of material below it in terms of temperature regulation.
Dunlop would have the advantage of a higher compression modulus (also called support factor) while Talalay would have the advantage of being more springy (in reference to your comment later in the question list). Because they are both available in a range of firmness levels … personal preference would be the deciding factor here. You are probably in the range where a 10 - 12 " mattress would have some benefits yes. Although I doubt you would “bottom out” on 9" … the extra thickness would be beneficial in terms of being more accommodating for larger body profiles or weights … especially with the larger “gaps” in side sleeping.
This too is a matter of personal preference. There are many people who believe that even memory foam is fine here although in general terms … more resilient materials are considered “better” for the other activities on a mattress because they tend to assist movement rather than restrict it. With memory foam in the top layers … you have a more restrictive or energy absorbing material in the mix so which if any layers you choose to “add back” some resilience would depend on preference. Some examples of adding some resilience to memory foam layers would be latex over them, latex under them, or Talalay latex (more springy then Duinlop) or innersprings as support layers. All of these are part of creating an overall feel which can be as individual as there are different preferences in people. When you are getting to a point of talking about fairly subjective perceptions or even a more subtle range of objective perceptions, then it’s important to have a reference point of personal experience with various options because words alone don’t really pass on how each combination may feel for you.
These are all good choices IMO. A few comments …
SleepEz: There is really no “down” side here and I would definitely talk with them at length.
FloBed: The down side here is really price/value, lack of ability to customize the top 2" layer, and the complexity and risk of their VZone which in it’s standard version is “reverse” zoned which can be risky (although it can be adjusted as well). They have great customer service as well though.
BayBed: I would give them a call to get a sense of their prices (which will depend on the custom layering you choose) but you will probably be happy that they are lower than your estimate.
Custom Sleep Design: In their case as well a phone call will give you the prices without having to jump through any hoops.
Berkeley Ergonomics: Because there is an outlet close to you … I would certainly include them in your more serious research as well as flyinion suggested.
In general … I have found that verbal communication with mattress manufacturers is much more effective than email or online communications or sometimes even information gathering. I’m not sure why this is so common in the industry but many of the best manufacturers (including some with some of the best service) are not as good with online information. Some of the reasons for this are because of the somewhat unique nature of the mattress industry in many ways but I think that this is partly because in most cases the “simple” questions that most consumers believe they are asking are often far more complex than they realize and many of the smaller and leaner manufacturers just don’t have the time to effectively deal with the time it takes or the complexity of written questions that have so many “it depends” in the answers. The better manufacturers will often do much better on the phone than they do with written communications in my experience.
You have some great options but for me the next steps would be mattress testing so that you have more personal experience and reference points and don’t have to rely on theory as much about the many different construction styles and “feels” that have been mentioned.
While Natura and PLB can be good for testing … they are not in the same value range as the others that were mentioned in your list in #6. This list is mostly latex though (mixed with springs in some cases) so you may also want to include some memory foam in your testing if you want to add some of the slow response feel to your choices once you’ve done some testing at BayBeds or BE.
There isn’t a single bad option on the list you mentioned and every one of them would make a good choice and provides great advice and service.
I think the “missing link” though is personal testing on the various combinations you have mentioned and this would require some personal testing so you can “translate” some of the theory into personal experience.
The outlets that (hopefully) are local enough that would allow you to test various combinations that have good value and I think highly of are the first 4 in the San Francisco list in post #2 here. In particular latex/innerspring combinations would be available at BayBed and Berkeley Ergonomics both of which I think highly of.
Besides these … Ergo Sleep Systems would have a wide range of mattresses available for testing but wouldn’t have the same ability to customize or be in the same value range. They do carry the PLB line including some of their hybrid mattresses which use combinations of fast and slow response latex which may be worthwhile adding to your experience.
I would first talk to each of the ones you are considering visiting along the lines of your post (telling them about your stats and what you are considering) to get a sense of what they have available that would be worthwhile testing and any missing information about pricing before you go there.
Once you have a reference point from local testing … then I would either choose the best local value (you have some good options here) or if you want to add online manufacturers to the mix I would spend some time with each of the ones you are considering on the phone. The ones you listed are more limited in memory foam options so if you want to include memory foam in the mix … I would test the Tempurpedic HD series for a reference point and then also include the two other online manufacturers in post #12 here who carry a range of memory foam mattresses to discuss what they have available in either memory foam or memory foam/latex hybrids.
So the next step IMO is local testing of various combinations of materials (with a phone call first) to help narrow down your choices of layering and materials.
Hopefully I answered all your questions but if I missed anything … feel free to post
Phoenix