Hi Stephtut,
Thanks for a great review and for sharing your ongoing experiences. I think they will be very helpful in helping others that are looking for the highest degree of customization possible for whatever reason this may be important and the process involved in getting there 
Once you decide on your final configuration … you could glue each side together and/or glue the top layer to the lower ones so they wouldn’t have any “independent” movement. This normally isn’t a performance issue but the tightness of the ticking/quilting (looser will “allow” this more because it isn’t as “squished” in the mattress but will also allow the feel of the latex to come through more), and the larger mattress sizes will also do this more. Of course it also makes it more difficult (but not impossible) to remove or replace layers down the road.
Yes … this would be the direction I would go with your “symptoms” but before I did this I would experiment with your pillows. Upper body and neck issues are often connected to the pillow you are using and this can make a significant contribution to the overall alignment of your sleeping system … especially with neck, shoulders, and upper body issues. There’s more about choosing a pillow and which may be most appropriate and some helpful links in the pillow thread here.
When you change your mattress … your sleeping profile will also change to some degree and a new pillow may be an important part of your new sleeping system. Keeping your head and neck in good alignment and well supported can make a big difference and while it can’t “cure” a pre-existing issue, it can certainly help as much as possible.
Thanks again for such an in depth and detailed review and sharing your ongoing process with us.
Phoenix