Hi TexMac,
That’s some kind of long distance mattress research! 
There is one factory direct manufacturer Furniture Row® Store Locations - Store Hours & Addresses which has two outlets in the area (Brownsville and Pharr) which makes two mostly latex mattresses (both have an inch of polyfoam in the upper layers which is OK) that may be worth including in your research (assuming they carry the latex models) before you start your travels.
Unglued layers are not an issue with shifting for the most part because latex is very “sticky” and will stay in place with a good ticking around it (unless you are moving the mattress or putting it on its side etc) so in those cases where shifting was an issue a zip cover makes it easy to re-align them.
Almost all talalay latex layers larger than twin XL have a glue seam (there are a few queen size molds but these are very uncommon) and many Dunlop layers will as well. You won’t feel these if they are glued correctly. They use (or should use) non toxic water based glue.
Post #2 here has some thoughts about split layering and how much it may be felt.
I’m sure looking forward to your feedback about your trip. You have some great choices included in your list.
Phoenix