Hi Sheelly,
With the 32 ILD core the mattress would be a little softer yet and while it’s not possible to know for sure because I can’t see you on the mattress or feel what you feel … it sounds to me like the mattress may not be keeping you in good alignment because it’s too soft for your back sleeping and your spine is curved too much and this may be causing your muscles to work too hard to keep your back in alignment. I would also agree with you that if the pain isn’t showing some signs of improving over the first week or two then I probably wouldn’t keep it for the full month.
That would be unfortunate because the materials are good quality even though it’s not the best value latex mattress that may be available to you and the design may not be the right one for you. At least a purchase from Costco allows you to take a chance on one of their mattresses and roll the dice for free because of their return policy which is about the only reason I would consider making a purchase from Costco (see post #4 here).
Of course all of this is “theory at a distance” but with the ILD’s you mentioned it’s very doubtful that the layers are too firm based on “averages”.
Phoenix