Hi summerkid,
Welcome to the Mattress Forum! 
Thank you for the compliment.
Unfortunately, it’s not possible to make specific suggestions or recommendations for a type of mattress from ESW for you because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) and your personal results on your current configuration (see mattress firmness/comfort levels in post #2 here).
As a general outline, there is some information about the many different pains/symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful.
Changing to the flexible slat system and finding appropriate adjustments for that system can be tricky, so I would follow the guidance offered by ESW in that area. If you’re moving toward the center of the mattress, that would tend to indicate that you desire a firmer deep alignment, as the slats meet in the center, and there is less flexing. But it could also simply be indicative that you just don’t like the flexing slat system, regardless of the comfort it provides. The moving toward the center is also curious combined with your comments that you feel the mattress is like a platform (hard) in your thoracic / low back area, yet moving toward the center of the mattress would actually make this harder overall (no flex in the area where the slats meet in the middle), so perhaps ESW would look at an adjustment of the flexible slats in this region for you. As far as the suggestion they are offering to the Nordic II HDM, this mattress has almost twice as much latex in the upper comfort layers as the Alpine II HDM, combined with an extra sect of microcoils, and I’m guessing that they are thinking that this will provide a bit more relief for the “firmness” you’re feeling in your thoracic/lumbar area (I’m not familiar with the specifications of each of their comfort levels). As some of your discomfort has been waning, it may also be that you have some learned alignment and you are slowly adjusting to a new mattress and this is getting slightly better as well.
With a bit more of a complex system like this, I would definitely work together with ESW and their suggestions, and impart as much information to them as you go through attempting to find a comfort that works best for you. I know it can be complicated when you are varying both the mattress and (potentially) the adjustable slat system. At the very least, you do have a 100 night refund policy should your purchase unfortunately not turn out as you had expected, but I’m hoping with some tweaking that you can find a comfort that you enjoy.
Phoenix