Hi hostage,
Your questions are fine – no worries.
There is no formula that can predict with any certainty what type of layering you may do best with that can possibly be more accurate than your own personal experience, and without this the next best way to decide on the design of a mattress is to use the “averages” and advice of a manufacturer for people of your weight range, body type, sleeping style, and personal preferences. These “averages” may vary on the specific designs and options that they have available, and in your case I would use your own testing on the three models available and then any comments you might have to the people at Jamestown about potential tweaks they might suggest in a custom product based upon your feedback. I wouldn’t consider the 12" Eurotop as overkill at all, and if you like a softer mattress I would potentially be more concerned with something that was too soft without adequate support.
It is a bit more common for the transition/upper layers of a mattress to be 2"-3", but certainly not always the case, and it’s not necessary for it to be that way. A 9" mattress using three 3" layers versus a solid 6" core with a 3" upper layer certainly does offer more options for customization. When you mention a denser ILD, I think you mean a higher ILD – a firmer foam? The uppermost layers within a mattress go through the most mechanical stress and break down the fastest, so if you were to replace a layer over time to wear, it most likely would be your uppermost layer of the mattress.
Durability depends upon many factors (oxidation, UV exposure, chemical exposure, mechanical shear forces, elongation, compression,…), but in general a higher ILD will be more durable than a lower ILD if tested going through the same amount of use and use patterns. Without going down the rabbit hole, that is the basic rule. And all latex in general is a very durable material.
Yes, it looks to be a solid 6" core in the specifications you provided and listed on their web site.
Yes, you would want to use something as breathable as possible, and you also may wish to consider a natural fiber protector as well. There’s more information about mattress protectors post #89 here, and here.
That’s too bad that your wife doesn’t want to assist you anymore, but maybe that will allow her to watch your child and then free you up to take more time testing out the mattresses at Jamestown. I would use their advice based upon your feedback after trying out their three products on the floor. One other thing you may wish to consider is using a plush topper, which I’m guessing Jamestown would have on hand, to do a little customization of your own on their showroom floor. For example, if you liked the plushness of the Eurotop but found that it wasn’t supportive enough, you could try their Firm model with a plush topper on it as an example of something with firmer deep support with extra softness on top. One of the key concerns I would have would be making sure you have adequate support in anything you choose.
Let me know how your testing goes.
Phoenix