Hi SeattleJen,
With 6" of latex toppers and another 1.5" of soft latex on top of your mattress it would certainly be enough for most people … even with your weight … that it would completely isolate them from the firmness of the base foam. Removing the 1.5" and having 6" vs 7.5 inches of soft latex probably wouldn’t make a great deal of difference for most people.
With higher weights it’s much more likely that you would be “going through” a single topper and the thin 1.5" layer of soft Dunlop and feeling more of the firmness of the support core underneath.
If you used just a 6" Dunlop mattress with both of your toppers it would likely work well and you would still have 12" of latex which would
be plenty. Thickness also adds to the feeling of softness and allows for the use of firmer foam and creates a more adaptable mattress for higher weights (see post #14 here).
About your options one by one …
It’s tough to argue with success so this would be a good option but it probably wouldn’t be that much different from option #3 which would also add the benefits of a lower height.
This would depend on the details of the softer mattress. If it involves the same firmness in the support core but thicker layers on top then it may work with just a single topper but if the softer mattress used a softer support core and had the same comfort layers on top I’d probably go with other options because the firmer support would probably be more suitable for you. You could always take a single topper into the store and test this option with one of your toppers (although I know it would seem odd to lug a topper into the store) to see how it worked. Overall this option seems less attractive than some of the others.
This is probably the option I would lean towards as well. It’s not likely you would notice a significant difference losing only 1.5" of soft latex but still having 6" remaining in the comfort layers (although depending on your sensitivity you may notice a little). Again … you could “lug” both of your toppers into the store and test the combination in person.
I’m somewhat surprised that you didn’t notice the firmness of the mattress when you were testing it in the store but at least you seem to have a good “handle” on the combination that works best for you.
Overall … I would tend to go with either option #1 or #3 and probably lean towards #3 because it has good odds of having the comfort you want and a little lower height.
Phoenix