Hi SkookumSleeper,
Welcome to the Mattress Forum! 
As you are in Vancouver, subject to first confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent (see this article) and to making sure that any mattress you are considering meets the quality/value guidelines here … the better options or possibilities I’m aware of in and around Vancouver, BC are listed in post #2 here. There is also a list of some online mattress sources for Canadians in post #21 here.
You seem to be curious about zoning, and various zoning systems can be very useful and worth considering for people who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article and in post #11 here. Also in post #2 here and post #7 here (latex momzone unique properties). While there are many “zoned” systems in the market, there aren’t too many all-latex ones. Two site members that use customizable zones include Flobeds (they are very experienced with their unique V-Zone system) and the Reverie Dream Mattresses (although they aren’t “zoned” in the more traditional sense of the word because they have cylinders that can be rearranged to provide different levels of firmness and feel).
Regarding comfort exchanges, while nothing has a 100% success rate, with a local purchase for the majority of people, careful testing using the guidelines in the tutorial rather than just testing for the more subjective “comfort” of a mattress (which often won’t predict how well you will sleep on a mattress or how it will “feel” when you sleep on it at home) and some good guidance from a knowledgeable and experienced retailer or manufacturer will usually result in a mattress choice that is well inside a suitable comfort/support range and will generally be “close enough” so that if any fine tuning is necessary it would be relatively minor and involve different mattress pads, sheets, mattress protectors, or perhaps even a topper if a mattress is too firm (see post #4 here and post #10 here). The cost of comfort exchange programs are built into the price of a mattress, so the decision of whether or not to offer such a program would be up to the retailer and their business model, and isn’t necessarily an issue of being able “to afford” such a program, but trusting in their expertise in helping their local customers find a mattress that suits their needs, and choosing not to build the price of such an exchange program into the price of all of their mattresses where everyone effectively subsides the few who might choose to exchange an item. Of course, if being able to return/exchange a product is an important part of your personal value equation, make sure to become completely familiar with such programs before finalizing any purchase.
Regarding a “compression index”, I’m a little stumped there. Perhaps you’re referring to the compression curve for latex foams? There is an article here (scroll to the bottom of the page – Talalay Comfort Zone) that summarizes it quite well. I’d advise anyone to avoid getting too technical with details and trying to predict how a mattress will feel, and instead trust your own personal testing and guidance from an educated and experienced manufacturer or retailer, as this will have a more practical application.
Regarding splitting layers, there would be little if any practical difference between two 2" support layers and a single 4" support layer if they were all the same type and blend of latex and the same ILD and were inside a tight fitting cover with 3" inches of latex of the same ILD on top of them (as an example). Two 2" layers would respond a little more independently and because the elasticity of the top 2" wouldn’t be connected and “pulling back” on the bottom 2" when it compresses and “in theory” it may act a little bit softer but in practical and real life terms most people wouldn’t notice any difference in terms of performance or firmness.
Also in “theory only” … two 2" layers that were exactly the same ILD as a single 4" layer could be less durable over the course of a long lifetime because they will act more independently and abrade each other slightly but I don’t think that any difference would be significant or even measurable in “real life” terms and the other factors that affect durability (see post #4 here) such as the firmness of the layers would play a much bigger role. It certainly wouldn’t be a concern of mine.
Multiple layers would have more options for fine tuning though both before and after a purchase because for example a support core with a medium over firm layer would be a little firmer than a medium layer and a little softer than a firm layer (closer to the medium) and you would also have the option to rearrange the layers to firm over medium which would still be in between a medium and firm 4" layer but closer to the firm. You could also use the softer top layer in the middle to create a firmer feel compared to having the soft on top. The main advantage of having more layers in other words is that for those who need it can provide more options to customize the layer combinations either before a purchase or by rearranging or exchanging layers after a purchase. While this is attractive to some people … it can also add some complexity that may not be necessary or may not justify any extra costs involved in having more layers for others.
Outside of a mattress with more layers having more options to customize the mattress, if a mattress is a good match for you, then one isn’t inherently any better than the other. And one of the advantages of trying mattresses locally is that you can try many different types and styles and combinations of materials and components and firmness levels and compare them to each other in “real time” based on your actual experience rather than just “theory” instead of trying one online mattress at a time and not knowing how it compares to the other mattresses that you could have tried or purchased instead.
I hope that answered your questions. If not, please let me know.
Phoenix