Hi Maddog29,
Welcome to the Mattress Forum! 
Unfortunately, it’s not possible for me to diagnose back pain causes online, as this can be very complex and there are too many unknowns and individual variable involved. However, there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here . Usually, back pain throughout the day is associated with a product that provides inadequate support, or isn’t a match for you in particular, but it can also be caused by a mattress that it too firm.
Regarding you photo, without actual landmarks being labelled (like in biomechanical analysis) and seeing in 3-D everything that is going on in your layers, it would also again be difficult for me to tell about your alignment. If is use your erector spinae as a guide, there certainly seems to be a lateral pelvic tilt and you can see where the lower thoracic “bends” into the upper lumbar region. It certainly would be plausible that this is the genesis of your low back pain issues.
There could be many reasons for this alignment. For the pelvic tilt, your legs may not be sinking in enough, causing a tilt. Or, your hips could not be sinking in enough, causing your waist are to drop and be unsupported. Alternately, you may be “sinking in” too much overall in this area and need a bit more surface support under your waist area. I’ve attached a photo here showing how both a too firm and a too plush mattress can cause these issues.
I’d be interested to see how you would respond with the “medium” version of this mattress and see if the slightly firmer latex on top made a difference. If that made an improvement, you would know that your surface comfort was too plush. I’d also be curious if you’d respond better to a more substantial (thicker) surface comfort layering, perhaps something along the line of plush over medium over the spring unit. I know this is not a combination offered by Luna – just me being curious, as a possibility is that your hips are sinking through the plush foam and then hitting the support of the spring unit and “stopping” at that area, where your lighter waist area is sinking into the plush surface comfort layer but your hips and shoulders aren’t sinking in enough to allow the comfort/spring to come up and “meet” your waist and support it properly.
You certainly could keep trying out the mattress and see if you adjust to it or if your back feels better, but at least you have some time to get used to it and you do have exchange/return options.
I’ll be interested to learn how things progress over time.
Phoenix