Hi Kelbrad7.
Welcome to our Mattress Forum. 
Thanks for sharing your sleeping stats and the DIY build you are considering. I can see you’ve received valuable insight from Mattrebuild as well.
This is entirely up to you. Some people purchase a layer of coconut coir or, as Mattrebuild stated, 1" to 1.5" of foam as a “stabilizing” layer. But this is not required or necessary.
It is impossible for anyone to tell with certainty which configuration is the best match for each sleeper as this is completely dependent on each of the sleeper’s BMI, body type, profile, and of their needs and preferences. Typically, a side sleeper in a normal BMI range can do well with anything between 2 - 4" comfort layer, Due to your husband’s higher BMI and sleeping position, he’d likely need more thickness in the comfort/transition layers as higher weights compress the top layers more deeply, they will also compress the deeper layers more so in cases where the top layers are thinner and the support core will also be more compressed “through” the upper layers then the density of the support core can also play a bigger role in durability than they would for lighter weights or with thicker comfort layers. There is more about the factors that can affect the durability and useful life of a mattress relative to different people (with different weight ranges and sleeping styles) in post #2 here and the posts it links to.
I’d make sure to ask for Sleep On Latex recommendations. When it comes to their own products SOL guidance is based on a great deal of experience and consumer feedback (which they take very seriously).
It depends on a few factors including how taut the cover is. Generally, the layers closest to your skin can have the greatest impact upon your comfort and can affect some of the properties of the comfort layer itself. E.g. depending upon the thickness and the weave and the blending of other materials, a cover using cotton can be stiff or barely noticeable. Thicker cotton covers can have a feel much like a canvas and not flex much at all, while others can have cotton blended with more elastic fibers to form a stretch-knit that is very flexible. Quilting wool into the covering will give it a bit more rigidity and some extra comfort, but it will tend to be on the slightly firmer end of the spectrum, especially as time goes on, as wool does tend to compress. This would of course depend upon the amount of wool used and how tightly it is quilted to the material. There could also be some benefits for temperature/moisture regulation with the wool, and a thicker cover quilted with wool can also provide a bit more protection in the long run versus oxidation/UV exposure for the latex. All in all, again Sleep on Latex will be your best source of information for cover questions
When reaching out to SOL, I’d make sure to provide them with good information about your body types, sleeping styles, and general preferences and history so they can best help you finesse your configuration.
Please keep us posted as you progress with your DIY.
Phoenix