Hi Hammer,
While adding a stabilization layer to the bottom of the spring unit would have the benefit of giving an evenly supportive surface for the coils to rest and sit on top of the foundation this would also add to the total cost of the mattress. For most sleepers of normal weight ranges not having a base layer between a slatted foundation and the spring unit will not create any issues but I’d still make sure that you covered all your basis and reach out directly to APM to get their perspective on the pros and cons of using a base layer and find out and about the reason(s) for not using one in their standard design. Arizona Premium is one of our Expert members of the site and I am sure they’ll be glad to clarify this for you.
Even though building your our own mattress may be a rewarding project (and you certainly seem to be having some fun with it … thanks for the visual element you added! 
) I would caution anyone who ventures on the DIY path that they may be lacking the materials engineering knowledge and/or mattress design/building/testing experience needed to design a suitable mattresses and I’d keep in mind that even though the materials and layers may be similar … any change in thickness/firmness or the way they are put and held together may alter dramatically the experience and the feel of the mattress as a whole.
Every individual layer and component in a mattress (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 asses the design and the specs of each mattress that uses exactly the same type of materials (Dunlop in this case, blended) in order to determine if that the mattress in its entirety will be a good “match” for you in terms of “comfort”, firmness, and PPP(Posture and alignment, Pressure relief, and your Personal preferences) This is especially important when you are trying to approximate the mattress you are considering with others that use the same type of materials and components which may be just as durable but have a different design or firmness level that may be completely unsuitable for you to sleep on.
Your design has 6" of latex on top of the Pocketed Coil system. While having 6" of Plush 14-19 ILD Dunlop for your wife’s side will certainly feel plush, this construction might not be supportive enough to keep her spine in neutral alignment and she may end in a hammock position. In general, given that the support layer is the same, the thickness of comfort/transition layers and their corresponding firmness are interdependent and must be correlated to ensure proper support. If the comfort/transition layers’ thickness increase, then their firmness would need increase and be adjusted to compensate for the added thickness. Generally, the main benefit of a thicker latex mattress is that it can be more adaptable for heavier weights and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body). While Avocado has 4" of comfort, your design for your wife’s side of the bed has 6" of soft foam. The difference between a 4" and a 6" comfort layer (and even 2" and 3" for the top layer) can be quite dramatic and also depends on a person’s weight, shape, and sleeping positions. Side sleepers will notice it more (they are more likely to go “through” a thinner comfort layer) and feel more of the properties of the layers below it.
Referencing Post #7 here about your experience with Avocado:
You are correct that the hips’ position in relation to the stiffer area is another variable that may influence how supported the lumbar area is. As she is quite petite then the shoulder area may also be impacted by repositioning herself on the mattress and may not solve the issue. Both you and your wife are within the normal BMI ranges (25 and 21) and the added thickness most likely would not benefit you and can be counterproductive so I would definitely recommend that you keep close to APM and discuss your proposed design to get their recommendations on a suitable ILD progression for your particular needs (especially as you already found your sweet spot in terms of comfort/support with the 2 x 2" of 14-19 ILD & 20-25ILD over the pocket coil).
In general, the thickness of a mattress or the number of layers or the thickness of any individual layers inside it is really just a side effect of the design and the design goals of a mattress and the thickness of any individual layers or the complete mattress is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person (see post #2 here ) and by itself isn’t particularly meaningful (seepost #2 here and post #14 here for more about the effect of thickness)
Based upon your comments and with some guidance from APM I think you’re on the right track with your DIY. I’ll look forward to learning about your progress.
Phoenix