Hi SoCal,
You are asking some great questions 
I think that questioning any opinions (including my own) is a healthy process because there are too many “expert” opinions on the internet (and other places) that don’t answer the “why” behind the “what” and are more self serving and designed to sell something than they are meant to educate. When ideas make sense to you and fit what I call the “preponderance of the evidence” (there may never be any absolutely correct opinions that fit all circumstances) … then you are in a much better position to make better choices that make sense to you.
All materials including latex will soften, degrade, oxidize, and break down over time so there is really no such thing as a material that won’t develop permanent impressions or soften under weight to some degree or over some period of time (unless you are talking about materials that aren’t suitable for use in a mattress which need the ability to compress and give under weight). The difference is in how long each material will take to do so and how long a mattress (or more accurately the materials inside the mattress) can maintain the degree of comfort and support that matched your needs and preferences in the first place. Latex as a category is the most durable of all the foam materials but even here there are many variables that will determine how long any specific mattress will last for you. There is more about the many factors involved in durability and how long a mattress may last for any particular person in post #2 here.
It may help to think of a mattress as a support layer on the bottom (which provides the the primary support of the mattress), comfort layers on top (which provides the primary pressure relief of the mattress) and then layers in the middle in some cases which are “transition” layers (or sometimes areas in the middle of the mattress rather than separate layers) which provide a little of both (they help with support and help with pressure relief) and make the transition between the firmer support layers and the softer comfort layers smoother and more gradual (rather than “going through” a comfort layer that is very soft and thin and feeling too much of the firmer support layers below it).
There are also differences between synthetic latex (SBR), natural latex (NR) and blends of the two as well as different types of latex manufacturing (Talalay and Dunlop process) which determine their performance and durability. you can see more about the differences between SBR and NR rubber in post #2 here (which has some reference links) and in post #2 here.
As you can see from these links … Synthetic Dunlop latex is not the most durable or best performing type of latex. If you were to choose an Embody mattress that was softer on top … then the upper synthetic Dunlop latex layers of the mattress would tend to soften and wear before the lower layers and when this led to the loss of comfort or support you would need to replace the whole mattress.
If on the other hand you chose the model that used a thinner and firmer layer of this same synthetic Dunlop latex … then it would make a good transition layer (not too soft and not too firm) and you could add a topper that used a higher quality, possibly more suitable, and more durable version of latex (or other material) as a comfort layer to customize your own comfort layer. This way the topper would increase the durability of the synthetic latex in the top of the Embody (the topper would absorb much of the regular compression forces). If the topper used a higher quality latex … it would also be more durable than synthetic Dunlop. In addition to this though … even a more durable version of latex that is soft enough for most people as a comfort layer can soften more rapidly than the deeper layers of the mattress (which are less subject to repeated compression) and if this happens or if your needs and preferences change over time … then all you have to do is change the topper rather than the whole mattress.
In other words … the suggestion of the firmest model (the Insight) was based on which model of the Embody would be most suitable as a base mattress in combination with a “custom topper” for use as a sleeping system which would be more durable as a whole and have more flexible choices and options available to you over both the short and long term.
While it won’t prevent a permanent indentation (nothing can do that over the course of time) … it will significantly lengthen the time it will take for deeper layers to develop soft spots or permanent impressions…
A good innerspring would also make a very durable and sound choice as a support layer in a latex hybrid mattress. While it won’t have the same performance characteristics as a latex core … a good innerspring is not likely to be the weak link of any mattress that has comfort layers on top that aren’t too thin (very thin comfort layers would lead to the the innersprings doing more of the work and this would affect their durability). In most cases … if you take a mattress that has “failed” … you will find the innersprings are still fine and it is the upper layers that are the problem. While it’s true that good quality firm latex would be a more durable support system than an innerspring … it would be rare that this increased durability would be a practical issue in a mattress when it is the comfort layers that are the weakest link. Generally the choice between a good quality innerspring and a latex core is one of performance and preference rather than one of durability (of course to some degree durability would also be an issue depending on someone’s weight and on the thickness, softness, and durability of the comfort layers). They are different in how they feel and respond to weight and compression. You can read a little more about the differences between an innerspring and a latex support system in post #2 here.
I’m guessing you mean any temperature regulating benefits of the gel foam (gel can also add to other qualities of a material such as compression modulus which is how quickly it gets firmer with deeper compression). This would depend entirely on what type of topper you used and on its cooling effect on the overall mattress in combination with all the other components and materials in your “sleeping system”. There are different types of cooling technologies (phase change, thermal conduction, and ventilation and humidity control) and if the topper (along with the mattress protector, sheets, and bedding) had better temperature regulating qualities than the gel memory foam in the optimum it would help with temperature regulation. If it had worse temperature regulating qualities than the gel memory foam in the optimum … it could detract from overall temperature regulation. It would also be worth noting that phase change materials are more effective “at a distance” than thermal conductive materials (which require closer contact). You can read more about the many factors involved in temperature regulation in post #16 here.
If you add a layer above memory foam … it will generally lower the ability of heat to reach the memory foam which will make it less responsive to temperature and firmer (or at least it will lengthen the time that the memory foam takes to soften under the heat of the body which can make it feel firmer). Different memory foams are more or less temperature sensitive so different memory foam formulations will be affected differently and to different degrees. They will all be affected though to different degrees. This can be a positive or a negative depending on the needs and preferences of each person and on the type of surface feel they prefer.
Gel memory foam is just a category of foam with many variations and versions in the category. You can see some of the different types of gel foams in post #2 here. If it is a more durable version of gel (which doesn’t have the larger particulates in it) and you are attracted to the other benefits of gel memory foam (such as temporary regulation of temperature until temperatures even out or the improved durability of some types or the improved support factor that gel can provide) … then they can make a good choice yes. I would base my choices on the knowledge of the supplier of any topper you are considering because if they are knowledgeable they will be able to tell you more about the specific benefits of the types of memory foams or gel memory foams they carry.
Some of the better sources for memory foam and other toppers or components are listed in post #4 here. I don’t recommend specific toppers (except to avoid the use of more thickness and softness than you need so there is less risk to alignment) because the choice of material is a personal preference that is unique to each individual. I would also suggest though that no matter what material you prefer in a topper … that you use the best quality and durability material possible because a topper will be the most likely weak link of a mattress (the top layer which is most subject to repeated compression, softening, and breakdown).
The thickness and softness of a topper that is most suitable would depend on many factory including your body type, sleeping positions, preferences, and on the mattress it was being added to and how much more softness you needed in addition to the pre-existing comfort layers in the mattress to reach your “ideal”.
In very subjective softness terms … you can go by 1" as “a touch to a little” more softness … 2" as “a little to a fair bit” more … and 3" as being a “fair bit to a lot” more. I would first sleep on a mattress without a topper to help decide any degree of additional softness I needed. Thinner toppers would be less pressure relieving (compared to thicker toppers of the same material) but would carry more risk for support alignment because they would put you further away from the support layers of the mattress. The key with toppers is “just enough” but no more.
Questions are always welcome … and if you have them then so will others
Phoenix