Hi mitenator.
Welcome to our Mattress Forum. 
Thanks for attaching the photos of your coils. It is hard to see if the coils are deformed or fatigued. Before jumping to conclusions I’d check the support system/foundation under your mattress. Whenever sagging or lack of alignment comes into play, I always suggest a “ground up” assessment to make sure that there is nothing under the mattress that may be contributing to the issue. Bed sizes above a twin should have good center support to the floor to prevent any flexing under the mattress and the people sleeping on it. In all cases … the mattress needs to rest on an evenly supportive base that will not sag or weaken over time under the weight of the mattress and the people on it.
You can verify if the support system you are using is appropriate if you place your mattress/spring unit directly upon the floor to see if that makes any difference. if you feel any improvement then it is possible that the support system is the cause … or … there is a combination of faulty/flexing foundation and coils being fatigued.
While generally, the spring units would be a durable component in a mattress it is possible that under certain conditions greater stress below the yield strength of the spring can cause microfractures or fatigue to occur. You may be able to “feel” if the coils are “donezo” by using the softer latex layer to see if you notice increased "bowing. I’d check this on different parts of the sleeping area to see the “feeling” persists in the middle vs the ends of the unit. You could stretch a string across the mattress to check if there is some indent with a ruler or place something like a yardstick across the mattress. This video gives a good idea of how to do this.
Good thinking! While there are benefits to a thicker mattress it is generally adopted by higher weight ranges sleepers (or a higher BMI) who do better with a mattress that is thicker than lower weight ranges but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of comfort, firmness, and PPP than it does on just the thickness itself. Thicker mattresses can also use firmer materials because thickness and softness are very related and work together.
You did not mention any of your stats but generally, a 10" to 12" mattress would be more than enough for most people so you can experiment with each of the layers individually and then combine them to see what works best. All the layers work together and the pocket coil is part of the general design of the mattress which is why I’d tend to focus on how well the mattress “as a whole” matches your specific needs and preferences in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences)
Please let me know how your diagnostic testing goes with the latex layers.
Phoenix