Hi Ray,
Welcome to the Mattress Forum! 
Nothing can replace your own careful personal testing, and as you have a good track record with your current latex mattress, I would use that as your point of reference. It sounds as if you wouldn’t necessarily have a need to go with more comfort layers, and you already have a decent idea of the types of layers that work well for you.
When ordering online, you should always give decent weight to the advice you receive from a manufacturer, as they have the most experience with their materials and how they best respond to different demands and populations, and they honestly have your best interests at heart and want to find the product/configuration that they offer which has the best chance at success. Sleep EZ is a site member here, which means I think highly of them and the advice hey provide.
With adjustable bed mattresses, I usually advise to keep the thickness topped at 12" or so. SleepEZ again would be the best reference for how their products perform at different thicknesses on an adjustable bed base, and since you are already doing well on a 10" thick mattress, there wouldn’t necessarily be a need to go with more layers unless you thought that you needed that additional comfort as compared to your current product. People with a higher BMI in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal, so staying with latex would certainly be an excellent choice for the quality of material. Post #3 here has more information and suggestions about heavier weights that is worth reading, in case you are interested.
I’m guessing you have the Rainforest model, which used three 3" layers of 100% NR Dunlop latex. If that served you well, there would be no compelling reason to switch to Talalay unless you were desiring a bit more of a “buoyant” feel in the upper layer and wanted a bit lower compression modulus (it wouldn’t firm up as fast when compressed) as compared to a similar Dunlop layer.
No, this is very common. I wouldn’t have a concern doing this.
Do you mean connecting the legs of the units together, such as their bed link kit 4B7410? I’m not sure if the distance between the holes (approximately 14") is accommodated with the Furniture Style S-Cape 2.0 leg spacing, and the leg joining kit is not listed as an option in the owner’s manual for this model. The base unit doesn’t tend to move around much once in position (even on a wood floor, where I’ll recommend using furniture cups under the legs), but I would phone Leggett directly to get a correct answer if you desired to use those. If by “connecting” the beds you instead meant linking for operating in tandem, that is accomplished wirelessly with these models.
Phoenix