Hi anemone,
You’ve clearly done some good research and have some good reference points although the exact ILD’s of all the mattresses you tested may not be available which would also be helpful in “translating” one design into another that is different.
I certainly understand the “fear” of any online choice but in the large majority of cases the first layer combination provides the options that are very close to ideal without needing any layer exchanges but of course it’s nice to know that an exchange is always an option. Of course you also have the return policy as backup so by the end of your first 90 days (when by far the huge majority of people have decided on their final configuration) you still won’t have to live with it if you are one of the very rare instances where no combination seems to work for you.
No … they have their “best price” every day and don’t have sales (at least that I’ve ever seen).
They certainly “allow” it (they will build whatever a customer wants) but they will strongly discourage it because it will have a very different “feel” on each side of the mattress because of the differences between how Dunlop and Talalay feel and respond. It won’t be an issue with layer thickness (all their layers are 3") but with the transition between the two sides. They also don’t recommend that the top layer is split without a quilted cover than can even out the transition between the two sides so a stretch knit cover may not be the best choice with a split top layer and a single top layer would generally work better (with any split layers underneath).
A thicker mattress with more layers can certainly provide more options (and in some cases more complexity) and will also change how the mattress feels and responds because thicker layers or a thicker mattress can be more adaptable to different body types and sleeping positions. There is more about some of the potential benefits of a thicker mattress and more layers in post #14 here and in post #2 here.
As you mentioned … the main difference between shredded latex and solid latex would be that the shredded latex can be “scrunched” more to accommodate different sleeping positions. For people that can sew it can also be opened to change the amount of the fill and then re-sewn if necessary. I don’t know the specifics of the pillows they offer (that would be better to ask them).
There is more about the tradeoffs involved in the different types of mattress protectors in post #89 here.
For most people … a protector is fine but for those that have more severe allergies then a complete encasement would do a better job controlling dust mites (and I would also focus on your pillow which can be a bigger source of dust mite issues than your mattress because it’s closer to your mouth and nose). There is more about controlling dust mites in post #2 here and more about mattress encasements for allergies in post #2 here. I would choose an encasement that was the same thickness or a little larger than the mattress (not thinner) or stretchy enough so the encasement doesn’t compress the mattress and create a “drum effect” or change the feel of the mattress.
There is always a tradeoff between waterproof and ventilation because anything waterproof will reduce the airflow and ventilation of the mattress which can lead to an increase in sleeping temperature for some people. With a cotton or synthetic “allergy” mattress encasement (without q waterproof membrane), many people would choose to use a separate protector on top of it to add the degree of “waterproofing” or “water resistance” they were comfortable with and that could be removed for cleaning but of course the more layers or bedding on top of the mattress the more it can affect the feel of the mattress. All of this is always a matter of tradeoffs between one “benefit” and another.
Phoenix