Hi Turquoise.
Good questions! 
When it comes to layering and type of foam or component being used in any particular mattress, the same general concepts apply to all different foams and where they are used in a mattress as comfort, transition, or support. This is more of an in-depth discussion and you’ can learn a little more about comfort (pressure-relief) and support, mattress primary or “deep” support and secondary or “surface” support in post #2 here and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in and in post #4 here that may also be helpful in clarifying the difference between “support”, “pressure relief”, “feel” and mattress design.
You are very observant … More individual layers would also give you more options to customize the “feel” and performance of the mattress either before or after a purchase which can be a benefit for some people that need to do some fine-tuning of their mattress. I’d add that there is much more to this than 2 vs 3 mattress layering when it comes to creating a new bed. There are many variables and criteria that manufacturers have to work with when designing new products some of which are more intangible in nature … such as “averages” of consumer’s personal perceptions or what we call PPP (Posture and alignment, Pressure relief, and Personal preferences)
With the emerging e-commerce brands, the increasing trend is to allow for more flexibility in fine-tuning a mattress to cover a wider range of needs and preferences. The long terms success of any new model (that has a wide enough consumer reach) will depend on the manufacturer’s experience, knowledge, and “educated intuition”, on how well consumers fit into consumer “averages” and even on the accuracy of interpreting the consumer feedback with the mattress or similar designs. Nobody has a “crystal ball” but some manufacturers are better than others and can better predict the outcome within a range of consumer needs.
You may wish to look at this step by step process of finding your “ideal” mattress in Post #10 here that can dramatically increase your odds that has better quality and value than anything you are likely to find with a major brand or typical mass-market outlet.
Yes, the latex used in each all latex mattress can often be a combination of either Talalay or Dunlop in organic, natural, blended, or synthetic, each having a different feel and price and most likely sourced from different suppliers. This again is connected with both the mattress design and pricing. Dunlop has a different “feel” and performance than Talalay and is less lively or springy. Some people prefer it to Talalay in the comfort layer. The support layer(s) being further away from your body contributes less to the general “feel” of the mattress and using Dunlop in the support core of the mattress where it has less impact and would not subtract much from the feel of someone wanting Tallaly, Dunlop is also less expensive. You can see a Dunlop / Talalay comparison in post #7 here but your own experience is really the only way to know which one you prefer with any certainty. Some people would notice more of a difference than others with transition or support layers that used each material if the top layers were the same type of latex because you will “feel” more of the upper layers than the deeper layers … at least when you first lie on a mattress. To this, you can also add the cost/price factor than the efficacy, performance and feels of the material used.
There are some videos linked in post #3 here that show the different production processes and again there is more about the differences between Talalay and Dunlop in post #7 here
It is important that you look closely at those variables and decide what is most important to you.
Hope this clarifies some of the differences and helps you move forward with your mattress selection. Looking forward to hearing back on your decision
Phoenix