Hi Gormeroth.
It is very encouraging when readers like you have such a thorough approach and analysis of choice, construction, and materials without getting trapped in marketing stories nor in the showroom feel of a mattress. (which usually is different than when you sleep for a while on the bed of your choice). Also, it’s great seeing how you take into account the various intangible elements and contributors that lead to your selection (including the psychological aspect and the power of habit). Research well-done! With your X-ray vision is hard to get things wrong.
I am glad that you are considering physical therapy or chiropractic adjustments in addition to a suitable mattress. This can make a world of difference when it comes to back pain and a bad mattress can certainly aggravate any condition you might have.
Even though you didn’t ultimately purchase the Tempurpedic mattress, you are correct that in the process of considering it you gained valuable data points and confirmed what you liked and don’t like about it …. especially how easy it can be to be swept off your feet with a nice comfy feeling that you can always return it. Once a purchase is made, you’d be surprised how many people can put up with minor or not so minor discomforts that could have been avoided with a more thorough approach. Once the purchase is made, life can take over, and psychologically it is perceived as a done deal. Many people try to get used to things that don’t quite fit and “make it” work …. this is not to the best approach as eventually, any small issue may slowly grow.
Another important data points. I am glad that you were able to actually go to a local shop, test, get good advice.
It’s great you were able to get the comfort layer memory foam density. Posh+Lavish doesn’t tend to provide much information about their mattresses, but this is a good starting point. As you well know every layer and component in a mattress can affect the feel and performance of every other layer and the mattress “as a whole”. I’d be a bit careful with trying to replicate the feel of this mattress because all the layers and materials aren’t the same and the design is different for each mattress, every difference between two mattresses can either have a cumulative effect or an offsetting effect that is very difficult for even a knowledgeable and experienced online manufacturer or retailer to predict how will play out. Most people only pay attention only to the ILD numbers of the foam layers and not to all the other specs or components that can make a significant difference in how a mattress feels and performs). … With this type of construction, I would normally suggest a thinner layer of memory foam over a thinner layer of latex so that you can notice more of the “feel” of both materials and there would be less risk of alignment issues.
11-12’ mattress can perform very well given a good comfort/support match. You can see my thoughts about the effects of thicker layers or a thicker mattress in post #14 . With a 12" mattress … the firmness would need to go up (than what you had for a less tick mattress) on average because thicker mattresses will “act” softer for most people. If you make changes to one of the specs (such as the layer thickness of the top layer) … then you may also need to make other changes to the other layers to compensate.
Another benefit of a thicker matter is that 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.
You seem to be well informed and know what to look out for in terms of specs, needs, and personal preferences. I am looking forward to your progress! The benefit of a DIY is that you can keep exploring and experiment until you find the perfect combination for your comfort/support needs, in a way that is hard to achieve with a “ready-made” mattress that also fits within your budget. 
Looking forward to updates as you go.
Phoenix.