Hi Denver Shoulder Pain,
The only way to know with any certainty would be based on your own actual sleeping experience because there isn’t any “theory at a distance” based on specs (yours or a mattress) that can take into account all the many variables involved in the hundreds of body types, sleeping position variations, weight distributions, preferences, health conditions, and individual sensitivities (see mattress firmness/comfort levels in post #2 here).
Part of the challenge you may have is if the issue is primary support (see post #4 here for more about primary support, secondary support, and pressure relief and how they are related) then any attempt to “fix” support layers that are too soft by adding layers on top will often only be partially or temporarily successful because it would be more of a “band aid” than a solution that “fixed” the core problem because the top layers can still “bend into” the softer layers below them and lead to alignment issues.
There is also more about the more common symptoms that many people may experience on a mattress and some of the underlying causes that may be involved in post #2 here and the posts it links to that may help with some of the detective work or trial and error that may be involved in solving the symptoms you are experiencing.
Your suggestion of adding two layers may be the “best possible” solution in the circumstances while keeping the softer core even if it isn’t an “ideal” solution. 6" of latex will reduce the effect of the softer bottom layer (they will absorb much of the compression forces before they reach the core) and you would have a reasonable comfort and transition layer but whether this would make “enough” of a difference is open to question and would depend on the person and the individual circumstances and there is really no way to predict whether it would be successful. It makes sense that it would move things in the right direction but I don’t know if it would move them enough.
If I was in your shoes I would be tempted to try sleeping directly on the soft core (with no other layers on top or on the bottom) with your quilted cover over the single layer (the cover will be too big) to see if this helps with your back issues (removing the bottom layer and sleeping directly on a thinner mattress can be more “supportive” and may provide better alignment) and if this seems to help the back issues (alignment) but still has some pressure issues then you can decide whether to add thinner layers on top to provide additional pressure relief. I would always try and find a “baseline” with what you have where you have good alignment as a first priority and then build “just enough” thickness and softness on top of this to relieve pressure in your most pressure prone position without going so thick or soft on top that you end up compromising alignment.
Phoenix