Hi swanlake,
You are asking some very complex questions and unfortunately there are no specific answers that can predict your experience with a certain mattress / topper combination with any degree of certainty outside of your own personal testing or experience.
The density of memory foam is mostly connected to it’s durability and not as much to how it “feels” … and each type of memory foam regardless of density uses a different chemical formula that can change it’s response, feel and performance compared to others (see post #9 here and post #8 here). Having said that … higher density memory foam often “feels” firmer (whether it actually is or not) and tends to be a little slower responding than lower density memory foams. If you are considering Christeli then it may be a good idea to talk with them and ask how their memory foam compares in different ways to the Tempurpedic version in terms of response time, temperature sensitivity, and overall firmness/softness if you are looking for a version of memory foam that is close to the feel and response of Tempurpedic (which tends to be more temperature sensitive and slower responding than many other memory foams).
In most cases … the best use of a topper is to increase the softness of the surface layers and improve pressure relief and not so much to improve support/alignment which is mostly a function of the deeper layers in a mattress in combination with the thickness and softness of the comfort layers. Thicker and softer comfort layers are generally more risky in terms of alignment … especially with memory foam. Memory foam can change in firmness over the course of the night and if the memory foam layers are too thick you can start off in good alignment but wake up out of alignment as the deeper layers can soften over time and as temperature “reaches” the deeper layers. You can read more about the relationship between primary and secondary support and pressure relief in post #2 here and post #8 here has some subjective guidelines for choosing a topper based on how much extra softness you believe your mattress needs.
The feel and response of a topper will also be affected by the type and condition of the materials that are in the top layers of your mattress because every layer of your sleeping system will have an effect on all the other layers to some degree. If there are soft spots or any sagging in your mattress that is causing alignment issues or if your mattress is already too soft then a topper generally isn’t the most effective solution to correct it. I would also keep in mind that back and especially stomach sleepers tend to do better with thinner / firmer comfort layers that let you sleep closer to the support layers of the mattress.
This may be because the TP comfort layer has softened more over time than because the heat is reaching it. Some of your body heat will reach it as well but it would take more time over the course of the night and would be to a lesser degree than if you were sleeping directly on the same layer. Some types of memory foam are also less temperature sensitive and change less with different temperatures.
You can read more about Christeli (they used to be called Luxepedic) in post #4 here.
Unless you are in a heavier weight range where durability is a bigger issue then some 4 lb memory foams can also make a very good topper choice that in some cases can be more effective than a 5 lb memory foam.
I know this probably doesn’t give you the specific answers you are looking for (because there are too many variables and unknowns for anyone to know with any certainty) but a topper choice depends more on “best judgement” based on the information you have available and your own experience on your mattress along with your confidence that the choice you make will be the most effective option based on more detailed conversations with each supplier than anything else. The exchange or return policies of a purchase can be an important consideration as well so that you still have options available if your choice doesn’t turn out to be ideal for you.
Phoenix