Hi rbev2308,
Changing the firmness of the support layers (the air bladders in an airbed) performs a different function than changing the firmness of the comfort layers in a mattress. The support layers are more for support and alignment (although they will also have a secondary effect on comfort/pressure relief) and the comfort layers are more about comfort/pressure relief (although they will also have a secondary effect on support/alignment).
If the issues you are having are related to the comfort layers (which are mostly about pressure relief) then changing the type, firmness, or thickness of the comfort layers can certainly make a difference (although you would be limited in terms of thickness by your cover). If a 3" comfort layer is thick enough for you (which is quite likely) then you could also use a firmer setting for your air bladder which is less likely to sag in the middle and could minimize alignment issues.
You can read a little more about the difference in “feel” between Dunlop and Talalay in post #7 here but in general it would be a preference choice not a “better worse” choice. Dunlop in a comfort layer will tend to feel firmer than the same thickness and ILD of Talalay but both are available in a range of firmness levels and the comparison only applies to the same firmness level. Either one can be softer or firmer than the other depending in the ILD of each layer.
Some manufacturers will allow you to order a single layer from them and then if you order a mattress from them at a later date will allow you to make the purchase without the layer you already have. SleepEZ is an example of this and it would also be worth checking with Brooklyn Bedding to see if they can do the same thing. This would allow you to use your topper as part of a component mattress purchase down the road.
There are too many unknowns, variables, and individual preferences involved to use “theory at a distance” or a formula to choose the thickness or firmness of a layer that would be most suitable for you with any certainty and your own testing experience on different thicknesses and firmness levels of latex would be the most reliable guideline but mattress firmness/comfort levels in post #2 here has some links with some generic information that may be helpful and the topper guidelines here (and one of the posts it links to) may be helpful as well. Most of the manufacturers that sell latex toppers also have the knowledge and experience to help you make a suitable choice and in some cases the topper can be exchanged for another firmness level or even returned. I know that Brooklyn Bedding allows for topper firmness exchanges for example.
If you are successful in choosing the right firmness for your topper (either initially or with an exchange) in combination with a firm setting for the air bladder then I think your plan could be a good one and you would provide you with the benefits of a latex comfort layer and give you some sense of what it’s like to sleep on latex and help you decide on how you may feel about an all latex mattress (or even a latex hybrid with a different type of support core).
Phoenix