Hey
Great to hear you`re aiming for Centurion - let`s get you there shall we
First: for Centurion idealy there is no gap between kidney/ab (for EIB 12.5mm) meaning you will need bigger shims. Also for Centurion there`s only one seam so sanding and polishing is a must but easy enough to do
By looking at the pics I´m with Luc that you should raise the ab plate and that will help with alignment.
Looking at this picture (ignore the blue lines for now) it seems as the ends of the back plate gets stuck on your body thus preventing the back to interact properly when you lift the kidney, causing the bunching you mention. I recommend a hot water bath to bend them slightly outwards.
Suggestion: after bending the back, start over with the strapping and I say first tape all the back pieces together using gaffa tape from the inside, put the tape close to the edges and two (or maybe just one) in the middle , as indicated by those crappy blue lines and with basicly no gap between the parts. Then see how it looks and feels when lifting the ab/cod to alligne with the top of the kidney as you can see below on Han (again - ignore that red line, not relevant here)
Good luck and keep us posted