Hi Searing.
Welcome to our Mattress Forum! 
Quite an adventure in the lands of “mattress surgery”! I see you are approaching this in the spirit of "adventure "which is always the best place to start. I am glad to hear that you have found both the forum and the site useful.
It’s worth mentioning that attempting to build something that is closely matching your previous mattress (when it felt “quite comfortable” can often a frustrating and futile attempt especially as you are considering different materials and also if you are trying to do this on your own, so I am glad you are looking for some guidance. I’d respond here to your initial questions but you’d be better off approaching one of our experts on their dedicated forum when it comes to deep dive in fine-tuning. You probably came across post #9 here the different ways that one mattress can “match” or “approximate” another one talking about several reasons why this can be difficult. In your case, the mattress rapid “aging” and breakdown of lower quality foams resulted in sagging. This happened over time which makes it difficult to pinpoint the moment when it turned from sleeping well to not sleeping so well and any ratios of break down/comfort. Also, within your 7 years, your body changed with normal aging and also tried to adapt and compensate for lack of support and alignment. (Your lower back pain in the morning is a good indicator of alignment issues).
Every individual layer and component you may choose for your mattress rebuild (including the cover, FR barrier, any quilting material, and of course all foam layers) will affect the feel and response of every other layer and component both above and below it and the mattress “as a whole” so you would need to carefully assess the design and the specs of each layer combination.
This is correct. The term I use is “virtual” or “phantom” body impressions, where impressions don’t show up when measured by someone testing for a warranty challenge, but the excessive softening of the foams in the area where you sleep is felt when lying upon the mattress.Virtual/ghost impressions in the mattress are caused by a premature breakdown of materials in your current mattress, and likely the culprit of your back pain.
The thickness of a mattress is much less important than the specifics of what is inside the mattress and which specific design is the best match for you in terms of PPP.(Posture and alignment, Pressure relief, and your Personal preferences) There is more in post #14 here about the potential benefits of thicker individual layers The mattress itself. 8" - 10" is inside the range for the thickness that would allow for a suitable design for the large majority of people.
The benefit to using latex in replacing the polyfoam layers of the original mattress is an increase in durability and responsiveness. It’s also a great material when considering lumbar and back support. As you’ve likely already read, latex is very elastic which gives it the ability to instantly form a pressure-relieving cradle that takes on the exact shape of the body. This means that in its softer ILD’s it has a pressure-relieving ability that is superior to most other materials and very similar to that of memory foam.
That said, higher densities of HD polyfoam or HR polyfoam can be a good, slightly less expensive alternative to latex in a core layer for those who are on a more restricted budget and are looking for a good quality foam mattress core.
Yes, this is a good support layer for a DIY mattress. Coils are rarely the weak link in a mattress. and a very good support layer.
You are comparing different products (Avocado & Arizona Pr) different thicknesses, components, and even foams of the same type and ILDs from different sources will feel different (given everything else is the same. There are many interconnected variables involved In building our own mattress and it is impossible to predict how a new layer choice would compare to your previous mattress even though the materials themselves may have similar properties they can be quite different in feel and firmness (for the same ILD) Dunlop and Talalay aren’t directly comparable in terms of firmness using only ILD numbers because there are several factors that can affect how soft or firm a mattress (or an individual layer) feels besides just the ILD of the material (see post #4 here )
I am not sure I understand correctly if the foam is on the sides, top, or both (your attached photos did not go through for some reason). Generally, provided that the fabric is strong enough to hold in place all components, removing the top foam should not be a problem however if on the side removing 3" of foam make the encasement lose and allow for the shifting of the layering.
Sometimes it is part of the design other times is making it thicker which in the eyes of the inexperienced mattress shopper equates with more comfort… reasons can vary. Theories are plenty but a mattress is only good as well it feels for any particular individual and as long as it maintains its comfort/support properties.
This is called a base/stabilization layer. Many mattresses do not have it and it is not an issue as long as the spring unit is intact and you have a good foundation under the unit.
If you chose to keep it here are some of the reasons, you’d want to have a stabilization layer
a. Provides a solid surface that enables the pocketed coil springs to “activate” and function efficiently.
b. Helps with stabilizing the mattress from any side sway.
c. Provides protection and functions as an insulator pad in foundations for cushioning steel and wood touchpoints, which makes the mattress compatible with most types of foundation designs.
d. It can be a cost-effective alternative to foam to increase mattress thickness
If you reuse it, I’d make sure is either in-tact or have it replaced. There are sources that sell 1" foam. You may want to check some of our Trusted Members that have 1" foams.
This is a good choice as soon as you decided on the overall thickness of your mattress. It allows for customization and any layer exchanges that may be necessary. Wool can have many effects on the latex underneath it depending on how thick, compressed, and/or densified it is and how soft it is relative to the foam underneath it. Over time as the wool compresses … it will have a greater effect on the foam underneath it as it becomes firmer. A wool mattress pad is also generally “softer” than a wool quilted mattress ticking because it isn’t an integral part of the mattress and will conform more to the layer underneath it. How well a wool quilted cover will conform to the latex underneath it also depends on the material that it is quilted to. The more stretchable the material (stretch knits for example as opposed to a damask cover which is woven) the better the latex can take on the shape of the body and the more pressure relieving the latex can be.
It depends if you keep the existing glued 1.5" foam and of its location. A good cover should be well taut with no slack to keep the layers secure in place. I’d take some good measurements with it taut if you decide to reuse it.
…. Without personal testing and going through the learning curve of becoming familiar enough with different types of materials and layering and how they work in combination with each other to predict how a certain layering may work for you … you would be much better off simply going with the suggestions of a knowledgeable retailer or manufacturer who is familiar with how the different options they offer fits the “averages” of people who may be similar to you.
Overall, there is no formula that can predict with any certainty what type of layering you may do best with that can possibly be more accurate than your own personal experience.
Hope it gives you some gear for the mattress adventure land.
Phoenix