Hi gpolo,
Your additional information seems to confirm my “suspicions”.
While the details of the Springwall are vague at best (they don’t say anything about the thickness, type, softness/firmness, or quality of the comfort layers) … the fact that it is “middle zoned” would be a benefit because this will help you sink down less in the middle area where most of your weight is. This would be especially important with back and stomach sleeping which need more support in the middle and/or thinner comfort layers.
In addition to this … the foams on top of these mattresses are probably lower quality and softer than the latex which means that they would “allow” your lighter shoulders and upper body to sink into the mattress more evenly. Latex gets firmer faster with compression than lower quality foam so it may be holding up your upper body too much while allowing your lower body to sink in relatively more. If you had a 2" soft comfort layer for example … your heavier parts would “reach” the firmer support layers more quickly and hold them up better while still “allowing” your upper body and shoulders to sink in enough for pressure relief.
The natural fiber mattress pad (assuming it is a slightly thicker pad rather than a thinner mattress protector) may also be “stopping” your shoulders and upper body from sinking in enough into the softer latex for your best alignment (it adds surface area or “padding” around your shoulders which reduces the force with which they compress the latex. If you spent more time on your side and this was your “natural” sleeping position then a thicker/softer comfort layer would probably be more appropriate but for shorter times spent in side sleeping then 2" would probably be fine and would be less risky for your other sleeping positions.
With the back pain in the mid/upper back area … this also points to comfort layers that are not allowing the lighter parts of your body to sink in enough relative to your hips/pelvis. As you mentioned … your choice of pillow can also make a difference with upper body pain but a pillow can’t “fix” mattress issues and a mattress layer can’t “fix” pillow issues so the pillow choice may need to wait until the “mattress issue” is improved.
To recap (to make sure I have it right) … your current mattress started with 3" layers of F/M Dunlop and then a 3" layer of softer Talalay on top.
You have switched this to having the F Dunlop/S Talalay layers on the bottom and the medium Dunlop on top which seems to have helped alignment somewhat.
You then exchanged the S Talalay for a M Talalay and are currently sleeping on F Dunlop / M dunlop / M Talalay and while this seems to have corrected any alignment issues … the upper layer is just too firm for good pressure relief (which is also connected to your lower weight which tends to need softer foam to sink in enough).
If this is correct … the first thing I would try is to remove the fiber mattress pad and try sleeping with something that is thin and stretchy on top of the mattress (such as a thinner stretchy blanket) without the thickness that can reduce the ability of the latex from forming a cradle for your upper body. While the latex may still be too firm … it could help to see if this moves you in the direction of better pressure relief.
If you still need some extra softness on top of the mattress … then rather than exchanging the medium talalay for something softer (which could contribute to your hips/pelvis once again sinking in too far and would only be going back to what you already had) … it may be helpful to just add a softer thinner topper (1-2") to help your shoulders and upper body to sink in a bit more without putting your heavier parts so far away from the firmer support layers below.
If it does turn out that a layer exchange would be helpful … it may even be for a firmer layer (such as medium Dunlop instead of Talalay) and then adding any extra softness you need with a thinner/softer topper.
The first step though is removing the mattress pad and using a stretchy blanket to see how this affects your experience.
One small step at a time and then a few nights experience on any change (to make sure that any changes are a pattern not just an anomaly) can help you do the best with what you have and decide on what you may need to add on top to bring you as close as possible to your ideal. 
Phoenix
I would also have this discussion with your Savvy Rest vendor to see if they have any other specific suggestions that may be helpful.