Hi DormiBene,
I think that one of the “dangers” of knowing more about mattresses (or anything else for that matter) is that we become more sensitive to the more subtle things that make a difference and “tolerating” things that we once thought of as “normal” is replaced with “adjusting” things in a quest for perfection that can be a side effect of more knowledge. Of course it leads to better choices and a much better and longer lasting sleeping surface but it also leads to challenges along the way that those who can “sleep on anything” or just “accept” things as they are don’t have to face. Knowledge can sometimes be dangerous … or frustrating
All the mattresses you were looking at were in the “medium” and “combo” range but the Crestwood probably has among the softest comfort layers of the alternatives you tried so I can see why you chose it. It will also soften a little in the first few weeks (as you know from lying on the floor model) and I would definitely wait for a while before adding anything which may not b necessary a few weeks down the road. The innerspring is firmer and more supportive than the Brentwood and because it also has less polyfoam and more latex in the comfort layers … it will soften less over time than the Brentwood. I think this is a good choice.
Your “20 ILD” layer from FBM is almost certainly not 20 ILD. What they are selling is highly unlikely to be what they are describing and is almost certainly Dunlop latex in a firmer ILD than they are advertising. It is also questionable if it is even high quality Dunlop. You can see more of my thoughts on them in post#2 here. In other words … it may be worth trying the FBM to see what happens but it’s effect would not be possible to predict because it is an “unknown” layer. It would certainly be the thinnest additional layer you could add so in terms of it’s thickness would be the least likely to affect support but it may also feel firmer and “dominate” the layers below it and any additional thickness using a foam can reduce support slightly by putting you further away from the support system of the mattress.
A latex topper should always be covered in a fabric that protects it from degradation from ozone and ultraviolet light which will degrade the latex much more quickly. Sheets or a mattress protector are not really effective or appropriate for this. In the short term while you are using it for testing it would probably be OK but in the longer term it will shorten the life of the latex. As you are mentioning … a thinner layer of latex can also be affected by rubbing especially if it is on top of the mattress. So a latex topper should be covered in a fabric and then used under both the mattress protector and the sheets.
I think that this is probably the best idea of all for the time being Once the first few weeks have passed and any “breaking in” of materials is done … then an evaluation can be more accurate in deciding if there is any fine tuning needed and what the best options may be.
Phoenix