Hi jsjs54,
You seem to have a good handle on things with your testing so there isn’t too much I can add that your testing hasn’t already told you 
This is a fairly common comment about latex because it has an unusual combination of softness, contouring, and resilience but is also very supportive and firmer when you sink into it more deeply.
This is also very true and the top layers of a mattress will have much more to do with pressure relief and the “feel” of the mattress when you first lie on it but the deeper layers can also make a difference in how you feel when you wake up in the morning (see post #4 here for more about primary and secondary support and their relationship to pressure relief).
The difference that layer thickness makes can also be surprising to many people because thicker layers can “act” softer than thinner layers in the same ILD. There is more about the different specs that can affect how soft a layer or a mattress feels (besides just ILD) in post #4 here.
If they are both using Talalay made in the US then they would both be from Latex International. There is more about the difference between 100% natural Talalay and blended Talalay in post #2 here but in very general terms 100% natural Talalay will be heavier (about 30%) and a little more “supportive” and blended Talalay will be a little more pressure relieving. The blended will also be more durable in the softer ILD’s than the 100% natural.
I’m looking forward to finding out how everything works out for you once you have put everything together and have had a chance to sleep on it.
Phoenix