Hi Mark9fiji,
If it turns out that you do best with an extra layer of soft latex it certainly would have fooled me 
Normally with lower back issues … the first thing I would suspect and the probability with the highest odds would be that the pelvis is sinking in too far. The first thing I would normally suggest in a case like this would be to try and firm up the deeper support. This usually involves putting firmer layers closer to the top (such as a firm or extra firm bottom layer in the middle). The main reason for this is that it would allow you to keep the soft top layer for pressure relief so you are only changing one variable at a time.
If changing the firmness of support layers is not successful … then the next adjustment I would normally make would be to use the second softest layer on top and to move the softest layer down one level. This would mean something like medium on top with soft underneath it with the firmest layer on the bottom. If the pelvis was still sinking in too far … I would put the soft on the bottom and the firm or extra firm in the middle with the medium on top. These steps will also lead to less sinking in of the pelvis but will also have a greater effect on the top pressure relieving layer as well which may introduce the possibility of pressure issues in some cases.
As I was testing all these steps … I would make sure I tested each of them for at least a few days (a week or longer would be preferable) and I would also keep an eye out for any symptoms appearing as a result of an changes in the comfort layer so I wasn’t trading one set of symptoms for another…
In your case though there are several unknowns and your experience could be contradicting the “highest probability” scenario.
First of all … after sleeping in your car for 20 months … I would suspect that your body has made some adjustments and it would take some time to get used to any new sleeping surface. Joints, ligaments, tendons, and muscles that are used to being in one position may need to loosen, stretch, or lengthen, and you may need to find a new sleeping style once again. When you sleep in more constricted spaces or on firmer surfaces … you normally won’t change positions as much or may sleep in different positions so your sleeping style may not be as “natural” as it would be on a sleeping surface that was more natural to you. Your body and it’s natural reflexes may need to “unlearn” this habit that has been developed as some type of compensation to a sleeping surface or conditions that are less than ideal. All of this means that it’s possible you may be on the longer end of a 90 day adjustment period rather than on the shorter end.
The other thing I noticed from your comments is that the thicker softer comfort layer seems to have solved the lower back issue but created an issue with your hip joint. Both of these may be because you are spending more time on your side and less on your back (now that you can) and when you do your hips may be flexing past their neutral position. This means that this is probably also an alignment issue (not a “pressure on muscle” or bone issue) because if you make the comfort layer thicker and softer it wouldn’t normally cause pressure issues. The back issue may be disappearing as part of the adjustment period as your body may be sleeping in other positions for longer periods of time which would tend to reduce issues that came from sleeping on your back even without any changes in your mattress.
So the long and the short of all of this is that your own personal experience is the most important thing and trumps any theory or ideas that someone else may have which are only a “theory” to be tested in real life.
Because of the time you spent in unusual sleeping circumstances in your car … I would take more time than usual to allow your body to adjust and pay most of your attention to “patterns” rather than the experience of a few nights. I would also pay attention to the gradual change of patterns such as certain symptoms getting worse or better over time.
So overall I would have guessed that your symptoms were the result of support layers that were a little too soft and/or comfort layers that were a little too thick/soft but if your experience indicates otherwise in the longer term then I would pay attention to what your body is telling you no matter what any “theory” may say.
As any doctor or health professional will tell you … our bodies are much too complex to use theory as a catch all for each individual person’s experience and it may take some detective work and even some trial and error to narrow down the cause of your symptoms to the combination of possible causes (your adjustment period, the break in period of the mattress, firmer/softer support layers, firmer/softer/ thicker/thinner comfort layers) that is the main reason behind them.
Basically … when you are dealing with symptoms that are connected with a mattress (to the degree that any set of symptoms are even connected to a mattress because there are always other possibilities that could be contributing to any set of symptoms as well) … you are dealing with a process that is the equivalent of a “differential diagnosis” which means that there are always multiple possibilities for any set of “symptoms” and the goal is to eliminate the possibilities one by one in the order that probability says they may be successful.
Each change is based on a “theory” that needs to be tested that probability says has reasonable odds of being successful. Your actual experience with each change and any change in your “symptoms” (better or worse) is the only way to know whether any theory or “hypothesis” and the change that was made to test it is the real “cause” and solution for your symptoms.
In other words … it is a process of educated and logical trial and error that takes each possibility step by step until the cause is determined.
At this stage … since you seem to be having some success with it … I would suggest continuing with the extra soft layer on top for a while until your “symptoms” (both in your changing sleeping patterns and the actual pain or discomfort) seem to have stabilized or seem at least to be a reasonable prediction of your ongoing longer term experience.
Phoenix