Hi Dioxine,
Welcome to the Mattress Forum! 
You are asking a myriad of questions and considering changing a multitude of variables and the questions you’re asking, while appearing simple on the surface, are actually quite complicated and unfortunately ultimately not something that I or anyone else can answer definitively for you. However, I hopefully can provide a bit of clarity and a few guidelines for you that might assist you and help you decide how to proceed (without getting too far down the rabbit hole). B)
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for combinations of materials or components because the first “rule” of mattress construction (and this is really what you’re attempting to do – construct your own mattress) is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
With that being said, we can start with a few bits of information you’ve provided. You’re using your air bladder as you “support core.” This by itself creates some issues, as air bladders are really “all or nothing” materials. You’ll sink in just a bit, and then you’ll be compressing the item fully (as you are inflating it at maximum pressure) and you’re surrounded by the fully stretched bladder. There is an absence of progressive resistance, and your heavier parts will sink down more rapidly until the stretched bladder stops further compression, at which point you’re rarely in good alignment. This is the weak point of air bladders.
To compensate for this, you have to use padding material on top of the air mattress. The support is being handled mostly by the materials on top of the air bladder, and when at the firmest position it is legitimate to question the usefulness or need of the air bladder at all. There’s a bit more detail about the dynamics of air beds here and here.
Since you’re relying upon the foam layers on top for most of the actual support, it would make sense to consider using some latex as part of your upper comfort layers, as it is an incredibly supportive foam, but at the same time still has a high degree of point elasticity. Latex also has a very good support factor.
You also complained about heat and the inability to reposition, both which tend to be characteristics of memory foam. Layers that are closer to the top sleeping surface will have the greatest impact upon comfort and sleeping temperature (including your mattress pad, sheets and linen). There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here.
Currently, you are using a total of 5" of memory foam, 2" of latex (which we assume is Talalay but there is latex on amazon that is advertised as Talalay which isn’t, and Talalay is usually rated by ILD and not density – a 4 lb. Talalay would generally be considered a “medium-firm” latex according to Talalay Global’s blended Talalay ratings, where a 4 lb. Dunlop would be considered more in the “soft” range), and a thick 2" fiber mattress cover.
Both the fiber mattress cover and the memory foam will contribute to your difficulty in repositioning, and they will also both contribute to a feeling of warmth (and the sinking you described when you wake up in the morning). If you’re wanting to keep using the memory foam, your best bet would be to use a smaller amount, the most dense that you have out of your pieces, and place that deeper in your mattress where it will have the least effect on your alignment as it sinks during the night and impacts your repositioning the least. Eliminating it completely might also be an option for you as you go through this DIY process.
Using some sort of latex for the upper layers might be beneficial to you and could address some of your concerns. Latex, specifically Talalay latex, will be breathable and supportive. and It will be easier to reposition with latex in the uppers layers than with memory foam.
You felt a bit better with the latex on the upper layer, which is pointing you in the direction of less memory foam, or at least what you use should be nearer the bottom of the mattress.
Your idea of using more latex toward the top of the mattress would be a common course of action for many people in your situation. You asked about the differences between Dunlop and Talalay, so you can read more about Dunlop vs Talalay latex in post #7 here. You can see more about the debate between SBR and NR rubber in post #2 here. I would consider all types of latex to be a high-quality and durable material.
Talalay latex is considered to be more “springy” or “lively” than Dunlop because you will generally sink in deeper which means there is more up and down “movement” or “ride” with Talalay and it springs back more strongly and quickly, and it is often preferred in the upper layers of a mattress. This creates a different “feel” between the two materials and is also why Talalay is often considered to be more pressure relieving than Dunlop because it allows for a deeper cradle in the same ILD … while Dunlop is considered to be more supportive because it doesn’t compress as deeply with greater weight … all else (including ILD) being equal of course. Some people will prefer the feel of one over another and because each has different firmness levels available … one is not “necessarily” better than another in either pressure relief or support layers if the right layering or ILD is chosen for each … but you may need to choose a softer ILD with Dunlop than with Talalay to get similar pressure relief in the comfort layers or a firmer Talalay vs Dunlop to get similar levels of support in the support layers. A “one step” difference in ILD which would be about 4 - 5 ILD would be “in the range” for most people where they felt similar. In a situation like yours, you may wish to consider a “softer” Talalay latex on the upper layer, along with a slightly “firmer” Dunlop layer beneath. A combination like this could potentially provide a combination that would provide good pressure point relief while side sleeping, but a decent firmness when sleeping upon your back, which you said you do sometimes.
I wouldn’t know what thickness for you to pursue. I would say that a 1" layer of Dunlop beneath your “Talalay” layer (again, we don’t know what this layer really is or the ILD) wouldn’t be enough to make a difference, so a 2" or 3" would generally be something for your to consider. Whatever you decide to do, I would pay attention to the potential return/exchange policy of any component supplier you’re considering, just in case things don’t turn out as you’d prefer. Post #4 here lists some component suppliers in case you haven’t found one yet that you like.
As what you’re doing is creating a DIY mattress out of separate components that are purchased from one or several different sources, I would refer you to post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well as you go through this process.
In the end, the only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it.
I know this is not the simple answer that you may have anticipated, but I’ve tried my best to “drill down” and makes things as simple as possible to help guide you through this process. And if you continue to use the memory foam on the uppermost layer of your product, you’ll more than likely continue to have repositioning and temperature issues. You may wish to experiment with not using that as part of your process.
Enjoy the adventure, and let me know how things progress for you.
Phoenix