Hi awesome23,
The Sealy Rivercrest (and most of their old springfree latex line) used several inches of polyfoam in the upper layers which is a much lower quality and less durable material than latex and would be the weak link of the mattress. It would be most of the reason that the comfort layers have softened or broken down. They are currently being sold as the Stearns & Foster Luxury Latex line.
There are a number of the manufacturers that are members of the site listed here that make latex mattresses that have split layering in a queen. Some local manufacturers will also build a split layer mattress as well. As dn mentioned all of these are a single mattress with a single cover but each half of the mattress is layered differently.
If you do decide to go with a single layer mattress I would tend to sleep on it first so you can use your actual experience to help you decide on a topper (there are some topper guidelines in post #8 here).
The first part of post #2 here also has some information about couples who have different needs and preferences.
I would also make sure that at the very least the top 6" of your mattress uses very high quality and durable materials to help accommodate your wife’s higher weight and prevent the foam in the mattress from softening or breaking down too quickly.
For some people yes and for some people no. There are many factors that can affect the sleeping temperature of a mattress (see post #2 here) and there is more in post #9 here about the cooling properties of different types of gel memory foam so it really depends on the specifics of the person, the overall design of the mattress, and on the specific type of gel memory foam being used. In most cases it will have some effect when you are first going to sleep until temperatures equalize.
Phoenix