Hi MAconi,
The type, thickness, and ILD of a latex layer that works best for you depends on your body type, sleeping style and personal preferences and on the other layers in your mattress. “Soft” is relative to each person’s perceptions and is very subjective.
Both 19 ILD and 14 ILD would be rather soft for your body type but it will also depend on the mattress design it’s used in.
If you are in doubt between two similar choices I would lean towards firmer rather than softer … and I would tend to use caution with 14 ILD unless it was a thin layer or your testing confirmed that you had a strong preference for it, and/or you were very light.
Yes the goal is good spinal alignment in all your sleeping positions and the means to get there is “support”. Some parts of your body that are heavier (such as the pelvis) need to be “stopped” from sinking in too far while others (such as the shoulders) need to be “allowed” to sink in for best alignment.
A few posts that may be worth reading since you are interested in “theory” are here and here and here and here and here which may help you connect a few more dots in how all the layers interact together.
Like the example of “floating in the air” and then slowly sinking into a mattress in the second link … you could sink into a mattress more deeply in every area of the body and still be in good alignment. If you only sink too deep (or not enough) in one area of your body you will be out of alignment.
Phoenix