Hi mike7,
The options of re-arranging or exchanging more layers allows for more adjustments of the mattress. You can read a little more about this in post #2 here.
I don’t know of any people who choose a 12" latex mattress that is suitable for their body type and sleeping positions that have had any issues with stability but it could also depend on the firmness and thickness of the layers. A mattress that thick with softer layers could be “jello like” to some people.
You can also read a little more about the potential benefits of a thicker mattress in post #14 here.
Several of the manufacturers or retailers in the general Los Angeles area list make or carry latex mattresses where you can choose your own layers including Foam Sweet Foam (better value), Ergocomfort, and PJSsleep and would be worthwhile visiting if they are within driving range.
Phoenix