Hi azrael,
Unfortunately the issues you are having are much too common because the major brands (including the Beautyrest Black line) use some low quality materials in their mattresses that are much more subject to premature foam softening and impressions.
Like many of the members that have come here with a similar situation you are in a somewhat difficult position where you need to exchange a mattress and are “locked in” to a store that sells many mattresses that I would normally suggest avoiding in the first place and where there may not be any particularly good quality/value options available to you but there are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation.
I would avoid any of the Serta mattresses unless you can confirm the quality/density of all the materials inside it for the same reasons as I would have avoided the Beautyrest Black so you don’t have similar issues in another year or two (see this article about the quality specs you will need). At the very least I would choose one of their firmer mattresses that had thinner comfort layers (less lower quality materials that are subject to soften or compress prematurely) and then you can add a higher quality and more durable soft topper to the mattress to add the additional softness and pressure relief that you would need.
At least the Tempurpedic mattresses generally use higher quality and more durable materials but they are also in a much higher price range than many other mattresses that use the same or better quality materials so they are not particularly good “value”.
While it’s not possible to quantify the sleeping temperature of a mattress or sleeping system for any particular person because there are so many variables involved and there is no standardized testing for temperature regulation with different combinations of materials … if your husband sleeps hot then I would also be cautious about choosing a memory foam or a gel memory foam mattress because they tend to sleep warmer than other types of mattresses or foam materials for many people. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Phoenix