Hi nms,
Thickness isn’t nearly as important as the specific design and the combination of materials that are inside the 12" although if you are comparing two mattresses that are otherwise identical except one has a thicker base layer then the thicker mattress will tend to “feel” a little softer (again for people who would notice the difference).
The support layers generally aren’t the “weak link” of a mattress and it’s true that Tempurpedic reduced the specs of their base foam to 1.5 lbs and for most people it wouldn’t be an issue. In higher weight ranges though where the base foam would be compressed more then it could be more of a factor and in this case someone may be better off choosing a mattress that used higher density foams and paying any extra cost that would be involved. All of this is part of all the tradeoffs that are involved in any mattress choice where PPP, cost, durability, and all the other parts of each person’s personal value equation that are important to that person will play a role in deciding on the “value” of a mattress purchase.
Return policies are all built into the cost of a mattress along with every other feature or option that is part of the mattress or the mattress purchase (such as free shipping etc.). You can see an example of the math in post #25 here. In effect the people who don’t return a mattress pay for the ones who do. The quality of the materials are always more important than a warranty IMO (you can read more about warranties here) and if there is defective construction or materials in a mattress it will tend to show up early in the life of a mattress.
You can also read more about the variables that are involved in durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to.
Phoenix