Hi trojanman,
You have certainly eliminated all the worst options, none of the mattresses you are considering have any weak links, and you are choosing between “good and good” in terms of quality and value. While I can’t help with what to choose when you reach this stage (this always depends on which of the many options and tradeoffs are most important to you), post #2 here may help you with how to choose when all your final options are good ones.
I’m not sure of the specifics of the fire barrier they use but if it’s an inherent viscose (rayon) / silica fabric then it wouldn’t be “chemical” (see post #2 here).
The easiest way to know would be to just ask Custom Comfort whether their Talalay is blended or 100% natural. You can read a little more about the pros and cons of the different types of latex in post #6 here. All Talalay latex has the same OekoTex standard 100 class 1 (safe for babies) certification for harmful substances and VOC’s.
I would also wonder about using a topper on a split twin adjustable bed if you planned to use each separately unless the limitations of the amount you could raise and lower the bed isn’t important because latex can tear easily. Of course if you plan to sync both sides and use them together then this wouldn’t be an issue and it would certainly reduce the feel of the split in between the two mattresses although you may still notice it to some degree.
The adjustable bed thread here may also be helpful as a “value reference”.
You have certainly done some great research and narrowed down your final choices to some great options and I’m looking forward to hearing about your final choice 
Phoenix