Hi cjwest,
I would be very careful with Stearns & Foster Luxury Latex line because in most cases there are thick layers of lower quality/density polyfoam in the comfort layers which would be the weak link of the mattress and you would actually be sleeping more on polyfoam than on latex. they are not a great latex choice not to mention their poor value.
The brand of a mattress means very little to me because a mattress is only as good as its construction and the quality of the materials inside it. Englander makes some good quality mattresses and some that I wouldn’t ever consider because of the low quality of the materials. They do make some Dunlop latex mattresses in some areas of the country that would be worth considering as long as you had all the information about what was inside it and the price represented good value compared to what else was available to you. They also make some memory foam and gel memory foam mattresses but knowing the specifics of what is inside any particular mattress you are considering is the only way to make meaningful quality and value comparisons.
I would strongly suggest that you never discuss a comfort choice or mattress choice through email but only over the phone which is a much more effective way of sharing all the details and more “nuanced” information that you may need to make the best possible choice or fot them to give you the best guidance. The advantage of a component mattress such as the Alexis is that you can either re-arrange or exchange a layer if your original choice is less than ideal.
Again … I would strongly encourage you to have these types of conversations on the phone … not with email.
All of your questions here would depend on the specifics of each material including the position, density, and thickness of any polyfoam layer. The base layer of a mattress though is not generally the weak link of a mattress though because a mattress tends to soften and break down from the top down so the upper layers of a mattress are the most important. You can read more about the factors involved in the relative durability and the useful life of a mattress in post #4 here and the other posts it links to. There is a LOT of misinformation (and misinformed salespeople) common in the industry. It’s quite likely that if you have spent more than a couple of hours on this site that you know more than 90% of the people that sell mattresses in the mainstream industry 
Phoenix