Hy Dymnd,
You probably have a little way to go before your mattress finishes going through any initial softening but as you mention this may not bring you into the pressure relieving range that you need (especially on your side). I also agree with you about the importance of surface tension and thinner or more stretchy mattress protectors will have less effect on the feel of the mattress than thicker or less stretchy protectors. Thicker mattress protectors(regardless of how stretchy they are) which can also have a much bigger “firming” effect on temperature sensitive foams than on other types of foam.
There are 3 different types of cooling technologies used in a mattress (which you can read about at the end of post #4 here) and of the three … phase change materials can have more of an effect when they are further away from the body than the other two (heat conductive materials and humidity and temperature controlling materials) so all of the cooling benefits of the phase change materials won’t be lost with a topper.
A topper will feel differently on different layers below it so testing a topper on a different mattress won’t necessarily provide you with an accurate idea of what the same topper will feel like on a different mattress.
Because of your focus on temperature and on some extra softness (should it be necessary after a little longer break in period) … I would tend to focus on a memory foam topper that is more breathable or uses some form of gel (either phase change or thermal conductive) … both of which will sleep cooler. Your idea about a thinner layer would also be a good idea because the more you sink into softer surface layers the more insulating a foam will be.
So I would be looking for a 4 or 5 lb memory foam topper (4 lb will tend to be softer) in the thickness range you are suggesting (maximum 2"). ILD is not really meaningful in memory foam because it’s softness is subject to time, heat, and humidity all of which will change the ILD of the memory foam. When you talk with a memory foam supplier … I would tell them you are looking for a faster response memory foam that is less temperature sensitive (doesn’t take as much time to soften), is breathable or cooling, and is in the softer range relative to other types of memory foam. This will give them the information they need to help you make your best choice. Gel memory foams that have the gel mixed with the memory foam as a liquid rather than particles are likely to be stronger and more durable than either particulate gel foam or a non gel memory foam of the same density (so lower than 4 lbs in this case would likely be less risky in terms of durability).
If you look a little further down in the same Post #4 which lists some suppliers … you will find some sources of gel memory foam (and other gel foams) in addition to the sources of memory foam without gel.
Phoenix