Hi mnmeater,
I think the first thing I would suggest is to "echo" what Padinn said which is to first give any new mattress or combination of layers a little more time if possibe. The initial adjustment period can take up to 90 days or so (although less is more common) and each change can also have an adjustment period involved as well as the body needs to catch up with the changes. Your mattress and each new combination will also go through an initial break in period during this time where any foam will go through its initial softening process.
In your case it may take some trial and error to find the best layering or material combinations for your specific circumstances because of the medical issues which aren't likely to be solved with a mattress and this makes things much more difficult and complex. You will also encounter a wide range of different "suggestions" from different people who all have differing opinions, thoughts, or experiences about what may help you best but as any health care practitioner will tell you each person is unique when it comes to medical challenges and what can work well for one may not work nearly as well for another even though their "symptoms" or circumstances may appear to be similar.
When you are healing then "going with what works best" temporarily even if it isn't the best choice for the longer term or for someone who doesn't have health issues can often be the best choice at least for a period of time. When your bone is broken then you need a cast temporarily until your bone has mended but once it has healed then what was "necessary" for the short term would become unsuitable afterwards.
What you are looking for is a mattress that will allow you to maintain the best possible and least painful sleeping experience for your circumstances and which relieves discomfort and pain to the degree possible while you are healing and this may also be different from a mattress which is best for you when there are no medical issues involved. Your own experience will end up being the best guideline because even "normal suggestions" may not be effective here.
The goal is generally to relieve tension and decompress the spine as much as possible so it can relax and recover from the stresses put on it during the day and the discs can rehydrate and recover and maintain their flexibility and cushioning between the vertebrae. Of course there is much more involved in healing back issues as well that involves all the muscles, ligaments, tendons, and all the other tissues that can be involved in any particular health issues. There is not a "simple" solution to any of this.
As far as memory foam vs latex or any other material ... it will depend as much on the design of the mattress as it would on the material in most cases because you will find one person that is well informed and has had good success helping people may make suggestions that will be different from someone else who is just as well informed and has had just as much success with completely different suggestions. Even two different doctors may have different experiences or ideas or treatment modalities and will have conflicting opinions or ideas about the same health issues.
Memory foam will tend to provide a more "stable" pressure relieving surface which tends to restrict movement more because it is much less resilient. This more stable surface can be an advantage for some who would benefit from less movement over the course of the night and need to be as "still" as possible. Latex on the other hand is much more resilient and can assist movement more which means that it may be less difficult to change positions which can reduce the stress of movement or sometimes reduce the risk of aggravating certain parts of the body because it takes less effort and muscle involvement and less "jerky" motions are required to change positions than with memory foam. Each has their own benefit and each can help maintain alignment with the right layering combinations so your own experience is the deciding factor. Your own judgement and insight about the differences and how you prefer to sleep (or feel best sleeping) may be the best "pointer" to a solution.
Overall though ... I think my
previous reply here
and other replies in this thread and any insights they may provide regarding whether your issues are connected with primary support, secondary support, or pressure relief are the best suggestions I can make on a forum where there are too many variables and unknowns and I don't have the medical experience or knowledge (or qualifications) to make specific suggestions for medical issues.
I would also consider using the "best" possible layering with the mattress you have (which appears to be 24 over 32 or 28 over 32) and then adding a 2" layer of 4 lb memory foam which will lower the resilience of the surface and create a little more "stability" without compromising your ability to move easily as much as a thicker layer of memory foam because it would be thin enough to "allow" some of the resilience of the latex to come through. If you use 28 over 32 (which appears to be a little too firm for good pressure relief on its own) then the extra 2' of memory foam may also soften it up enough that the combination can provide both good pressure relief, good stability, and good support underneath.
If you did decide to go with 4" of memory foam (which would generally be somewhat "risky" especially with softer layers that are already underneath and as your doctor mentioned could create "pelvic tilt" because of the thickness and softness of the top layers) ... then I would use it over the firmest latex layering that was available to you or even over the single firmest layer if it fit in the cover. You also have the choice of using 3" of memory foam which could be used to replace one of the latex layers.
Of course these are "educated guesses" and your own experience may completely contradict this but if you are able to keep the layers you have for a short time until you can purchase a softer memory foam topper then this would give you some room to experiment with latex/memory foam combinations. The 8 lb memory foam would also be a good choice because it is very soft and conforming once it has softened and is more "supportive" than lower density memory foam but you have good support under the memory foam anyway and the 4 lb as they mentioned would be more breathable and also less risky in terms of cost if it doesn't work as well as you hope.
Phoenix