Hi dfwjeff,
I think your logic is “impeccable” 
The thinner top layer would create a firmer surface where you didn’t sink in quite as much while the talalay lower layers would allow for slightly more sinking in and softness and will “help” the comfort layer more compared to the same ILD with Dunlop.
The only thing where “theory” may not quite match reality though is that the ILD’s may not be quite the same between manufacturers which would make a difference and latex … especially Dunlop latex … doesn’t come in exact ILD’s at the best of times anyway so ILD matching is approximate.
As you mentioned … the thinner top layer also leaves you with more options if it isn’t quite right with the addition of various types of toppers (probably 1-2" depending on type) based on your specific experiences on the base mattress. I would leave the choice of which topper is best (if its needed at all) until after you have had some consistent sleeping experience on the mattress that can give you better guidance about the type of adjustments or fine tuning that may be necessary.
I doubt that the complexities of zoning will be necessary but if you are willing to take on the challenge of some mattress design then the only thing I would be careful with is where the zones begin and end. In general … the middle third of the mattress would need to be firmer while the upper and lower third would normally be softer (rather than a half and half split). Most of the heavier hips and pelvis area and the shoulders would be in the upper half of the mattress if you did a zoning split in the middle and if you are doing a two zone mattress then the softer zone shouldn’t go beyond a couple of inches above the belly button or near the bottom of the ribcage.
Another option would be a tension adjustable foundation (you can read about them in the foundation thread here) which are usually more expensive but there is a lower cost version here which can also provide some fine tuning options which can allow the shoulders to sink in a little more if necessary.
Phoenix