Hi DanielH,
That makes a lot more sense to me and helps point to some possible changes ... and I think they are in the same direction as you are thinking. For some reason as well I thought you spent some time on your stomach but in re-reading the post I see you don't which is a good thing because stomach sleeping would make it more important to use as thin a comfort layer as possible.
The lower back pain usually points to pelvis sinking in too far. The other possibility is a "twisting" of the lower body into a half stomach half side sleeping position but I would suspect that in your case it's the first one.
The lack of resistance when you slide your hand under the gaps also points to the need for a thicker comfort layer so that the foam underneath the small of your back can fill in and make firmer contact with the body. This same increase in thickness would also improve shoulder pressure relief.
Using softer foam may also further improve pressure issues in the upper body if you thought it was necessary to improve it more than an extra inch would do so.
Of course the trick is to find how much of a change in either thickness or firmness has the best odds.
So bearing in mind that I can't actually feel what you feel .. here's what I would tend towards.
I would increase the thickness of the comfort layer by an inch and use the same ILD. This would improve the "gap" support and improve the upper body pressure relief (there would be more foam for the shoulders to sink into).
I would increase the firmness of the support layer to 40 ILD and reduce the thickness to 4" (so the height of each side would be the same). This thinner layer would further increase the firmness of the base foam but I suspect this would be OK because the thicker comfort layer would act to better insulate you from the firmness of the support layer. This also leaves open the possibility of adding a further 2" or so of foam on the bottom if the overall thickness of either your support layer or the mattress itself wasn't enough and couldn't conform to the different positions as well as you wanted or you felt that you were bottoming out.
The only other possibility would be to decrease the firmness of the comfort layer so that you had 3" of 15 ILD. 1515 foam is a very common foam used in comfort layers (meaning 1.5 lb density and 15 ILD) and the advantage of using a 15 ILD in a denser version is it would get firmer faster than the 1.5 lb foam and act to insulate you better from the firmness of the comfort layer. Another common foam used in the comfort layers of some manufacturers is 1824 meaning 1.8 lb polyfoam and 24 ILD but this would be used more by local manufacturers and not so much mass market mattresses which tend to use softer foams. This is just to give you a comparison between your mattress and some "typical" feels.
Hope this helps and your feedback has been very helpful.
For adding the image ...
Step 1: Click add file and a window will open.
Step 2: Browse to the image you want to add and double click it or single click and click open (making sure it's an allowable file format).
Step 3: Put your mouse pointer on the very bottom of the "insert" box (to the right of the "remove file" box) and it will turn into a hand (if you put the pointer in the middle of the "Insert" box it will stay as an arrow and will open up a new window if you click it which you don't want). Left click when the hand is visible.
The file should be inserted (like the one I added)
If you just complete step 2 and then don't click insert and click submit instead ... the file will be attached.
Keep the questions coming until you have a good sense of confidence about where to go next.
Phoenix