Your post brings up some very interesting concepts which are part of the “art and science” of mattress construction and how different materials interact differently with the many different body types and sleeping positions that are very individual.
Alignment itself involves different factors. The most important is spinal alignment from top to bottom of the spine and maintaining the natural curvature of the spine however it also involves “side to side” and “rotational” alignment and the alignment of the joints all of which can have a “natural” or “neutral” position along with a “learned” position. All of these interact with the ability of a layer to re-distribute weight throughout the surface of the body in each sleeping position (and there are many variations of the 3 basic positions).
An example of a “learned” position is people who over the course of many years have become used to a more “hunched” posture or a posture where the shoulders are more forward than back. This creates a learned position and in these cases a more “correct” position can actually be less comfortable or cause pain or discomfort.
A soft pillowtop will “allow” these types of positions more easily because of the characteristics of the foam. Latex has a higher compression modulus or"sag factor" than polyfoam and also a higher resiliency (stores more energy instead of absorbing and dispersing it and “bounces back” more readily). What this means is that on latex … people who have good posture both top to bottom and side to side will be more supported in this “good posture”. Those who have a more “learned” posture (or even a natural posture or body type or weight distribution that is outside of the “norm”) may have a feeling that the latex is “pushing back” as it tries to even out the alignment in all directions. Polyfoam will “allow” more sagging into the material. This feeling of “pushback” as many people call it will be on the areas that are more “hunched” or need to sink in more relative to the lighter areas of the body.
This has been the subject of many conversations I have had with various people (including various chiropractors and other “experts” in medical, health, or training professions) about the difference between sleeping on a mattress that provides a theoretical “best” alignment which for some people may be uncomfortable because it is different from the “learned” position and involves stretching muscles and ligaments and tendons that have “tightened” in various positions over the years vs the benefits of a mattress that may not provide “perfect” alignment but are more comfortable and closer to the “learned” position. The general consensus is usually along the lines of “somewhere in between” often works better because it can encourage a sleeping position closer to natural alignment but not in such large steps that the process is too uncomfortable and the body has smaller adjustments to make.
So in these cases (or in cases where the natural alignment of the spine or the body’s weight distribution is outside of the norm for other reasons), then the challenge becomes how to find the “in between” that works best for each individual.
In your case there is another issue which is the shoulder soreness on your side and tossing and turning which would be related but more directly connected to pressure relief. This generally indicates a comfort layer that is either too firm or too thin to allow the lighter wider (relative to the pelvis/hips) shoulders to sink in far enough for good pressure relief (which can also lead to upper body alignment issues). A secondary cause for this could be a pillow which doesn’t support the natural curves and weight of neck and head and some of this weight is shifted to the shoulders.
Because these types of situations may involve some experimentation because there are so many variables that can be unique to each person … I would generally suggest experimentation with less expensive components or materials or materials that are refundable so that you don’t take on the additional expense of buying a topper which would work well “in theory” but turns out not to quite match your unique circumstances.
In terms of thickness … since you already have 3" of 28 ILD latex and a quilting layer over this … I would tend towards a thickness that can fine tune this more than provide what you need all by itself. This means that I would suggest a thickness in the range of 2" or so (perhaps a little less) to provide a little more softness and thickness for your shoulders to more easily sink into. The challenge here is to make sure that it is not so thick that you are too far away from the support layers so that your hips/pelvis will “travel” too far before being “stopped” by the support layers without the shoulders and upper body being “allowed” to do the same thing. In other words if your pelvis is sinking in more and your shoulders upper back are being help up higher … then relative alignment will suffer. If “both” your hips and shoulders travel “through” the topper fairly evenly … then relative alignment will still be good. With the 28 ILD I would suspect that your pelvis/hips are heavy enough to go through it and are being “stopped” quickly enough by the support layers that your pelvic tilt and alignment in the lower back area is good (thus the lower back pain going away) but that this combination isn’t allowing your shoulders to sink in far enough and needs some “plushness” to that it can also “go through” the upper layers more easily and not be “stopped” quite so quickly. This would also allow your upper back to “sag” a little more into it’s more comfortable learned position because of an extra layer of softer and perhaps less resilient material with a lower sag factor.
If softness/thickness alone was the only issue … then a layer of lower ILD latex would likely work well (in the 19 - 24 range). 19 of course would be softer and if the weight and width difference between your shoulders and lower body was more than average then 19 may be better because it would allow both the upper and lower body to sink in a little deeper and more evenly instead of only the heavier part. The more “normal” suggestion for greater weights would be a little firmer but this may “hold up” the shoulders more than the lower body. Softer would also be closer to the “pillowtop” experience.
But even a lower ILD latex still has a higher compression modulus and resilience so it will still “encourage” better alignment and you may still feel some “pushback” even though it may be less … it will just happen a little deeper in the mattress. Because of this … I would be tempted to try a polyfoam topper because it has a lower sag factor, is closer to what you are used to, and is a less expensive way to experiment. If it turns out that this “fixes” the problem … then you would have a good sense of the thickness and softness level that would work and the next “incremental” step could be the purchase of a latex topper in a comparable softness level.
So I would be tempted to try a topper in the 1.5 to 2" range that was soft polyfoam (not so durable but good for experimentation purposes and replaceable with latex when it wears out) and refundable if it didn’t work.
Other options that could also help would be wool (which is less resilient than either polyfoam or latex and will provide more “localized” pressure relief and has other benefits such as ventilation as well) or memory foam (which has a completely different slow response “feel” and could be a bit more “risky” in terms of alignment) which would both allow the upper and lower body to sink in relatively evenly but these are more expensive to “play with”, have a different “feel” and are not quite the same as the pillowtop “feel” you are likely used to. My longer term preferences would lean towards soft latex or wool but but some preliminary experimentation would make good steps and provide some helpful feedback towards this “longer term” and higher quality solution (or possibility).
So something that is inexpensive and refundable like this may be a good first step as part of a “fine tuning” experiment. How well this works (or doesn’t) can provide some good feedback about what the next step could be.
Hope this helps.
PS: none of this takes into account how any changes will affect your girlfriend who it seems is quite happy with the comfort and performance of the mattress as it is so if she asks me I will deny any responsibility for any suggestions that make things worse for her