Hi Parran,
I’m not quite clear on the exact layering of the mattress you have but my tendency would tend towards the opinions of the supervisor.
Their site indicates that you have a choice of different comfort layers (4 different options) but doesn’t mention any specifics of the choices available to you for the 6" Dunlop core.
It would be helpful to clarify what the layering of your current mattress is and the exchange options available to you.
What is the firmness/ILD of your current Talalay 3" comfort layer?
What is the firmness/ILD of your current Dunlop 6" support layer?
What are the firmness options available to you for the 6" Dunlop core (they don’t list this on the site)?
Some general comments that may be helpful in the meantime …
Every mattress is a balance between the conflicting needs of pressure relief and support.
Softer layers will generally feel firmer to people that have lighter weights because they don’t sink into the layers as deeply as heavier people.
Comfort (pressure relief) comes from a combination of the softness and the thickness of the comfort layers. Both thicker and softer layers allow you to sink in more deeply which increases pressure relief.
Support (spinal alignment) comes from a combination of thinner comfort layers and firmer support layers both of which will “stop” the heavier parts of the body from sinking in too deeply (causing misalignment). With your lower weiht … a thinner soft comfort layer may be all you need (as long as ther suupport layers weren’t too firm) for both good pressure relief and good alignment.
With lower back pain … the cause is generally that the hips/pelvis is sinking in too far and this could require either thinner comfort layers of firmer support layers.
I see no reason to add a firm Talalay middle layer when it would perform the same function as a firmer support core except it would add to the cost of your mattress. At your weight there would be no reason to buy 12" of latex of any type IMO.
While I understand that it is not free … their layer exchange costs are capped at $45 and their return costs are capped at $99 so at least this is reasonable. There are also other online latex mattress manufacturers that have a wider range of layering and/or material options available that could save you more than either of these if it became necessary. They are listed in post #21 here.
This is the nature of latex which is very elastic and “point elastic” which means it will compress more under the more concentrated weight of sitting because it compresses more “exactly” under the weight with less effect on the material around the weight. Your weight profile when you are lying down is much different from when you are sitting and what happens when you sit on the mattress is not an indication of its support when you are lying down.
Latex is not heat sensitive so an electric blanket would be fine. This is unlike memory foam which is heat sensitive where an electric blanket is not a good idea.
So hopefully this helps and some clarification about the layers that you have and the options available to you may also be helpful.
Phoenix