Hi Tennisman,
I can certainly understand this. If someone had told me a few years ago that I would be spending 16 hours a day on a mattress site and forum I would have told them they were crazy … yet here I am … and loving every moment (well mostly anyway) 
If you are comparing a DIY to another mattress using the same materials that doesn’t have the same options for re-arranging and exchanging then I don’t see a down side no and I would say it would be a value bonus. In other words if I had a choice between a mattress with the same materials that had all glued layers and the same cover and both were being sold online at the same price then I would personally choose the DIY version. The only potential down side would be that layers may shift a bit over time but this would be easy to “fix” by zipping open the cover and shifting them back … if it even happened at all. You would also have the option of gluing the layers yourself if that was a preference but you would lose the benefit of being able to change out the foam layers down the road if your needs or preferences changed or if any of the layers needed replacing before the others. Overall … all else being equal … I would treat the ability to rearrange and/or exchange layers as a value bonus.
In terms of the suitability of any particular design (as opposed to its quality or “value”) for your particular body type or sleeping style … it is always better to work directly with the manufacturer themselves who are much more familiar with how each of their options may work for different groups of people based on the “averages” of their customers with similar circumstances. personal testing on mattresses that use similar components and materials can also be very helpful here in providing a guideline for an online choice.
For you … 3" of 5 lb foam would allow some of the firmness of the layers below to “come through” when you were sleeping on your side because 3" is a little on the thin side by itself for a side sleeper that was in your weght range. in other words … the next layer down would be more of a transition layer that contributed to both the comfort layer and the support layer of the mattress.
For her lighter body weight it would be more suitable as a comfort layer by itself because her lighter weight wouldn’t “go through” the comfort layer as easily and would feel less of the layers below it. This is all part of the challenge of a DIY. I would tend to test similar mattress designs on a local basis as a guideline. Generally a mattress with 3" of higher density 5 lb memory foam will tend to be on the firm side so this may need to be modified by your choice of the layer below it.
Slower response memory foams can tend to sleep warmer than faster response lower density memory foams and the gel foams do tend to be a little more breathable and cooler than most of the slower higher density memory foams available. They will also have some 'temporary" benefit that comes from the gel itself because of the thermal conductivity of the gel (which is why it generally feels a little cooler when you put your hand on it) but once the temperatures equalize then the foam itself is an insulator and the temperature benefits of gel are reduced or disappear. There are also many other factors involved in the sleeping temperature of a mattress besides just the memory foam itself (such as the quilting, the ticking, the mattress protector, the sheets you use, your bedding and bedclothes, and environmental conditions) all of which have a combined effect on sleeping temperature. In many cases one of these layers that tend to sleep warmer or cooler can be offset by the other layers that are part of temperature regulation. There is more about temperature regulation in post #2 here and about sleeping microclimate in post #29 here.
Overall … I like the idea of a DIY where you have options that aren’t available with a more traditional mattress but I would do some local testing on similar layering and make sure you have a more detailed conversation with the manufacturer as a way to lower the risk so that you are making the best possible initial choice and the odds are better that your mattress will only need some fine tuning that can be accommodated with re-arranging or exchanging a layer rather than a more fundamental change in design.
Phoenix