Hi jujub1,
You really have two key questions here … one of which I’ve answered and one which I haven’t. I’ll reply to them one at a time to make my thoughts a little clearer.
Question #1
Any mattress (including the PLB) can be made to be as soft as you wish by adding more and more soft layers on top of the mattress using various different foams and other materials. The challenge with this is that the thicker and softer the upper layers are … the more you risk affecting your spinal alignment (which can cause back issues). In your case however it seems that pressure relief is a bigger priority than alignment however I would still make sure that your alignment was as good as possible which means using the minimum amount of softer materials on top of the mattress that provide the pressure relief you need. “Just enough” is the key here.
The key with this is to add a layer and use it for a while (an adjustment and evaluation period) then decide if this is “just enough” or whether you need an additional layer of something even softer. the choice of additional layers will depend on how much more “softness” and pressure relief you believe you need after each evaluation and adjustment period.
Question #2
This is the one I tried to answer previously. I’ll try to be a little more clear this time 
Because you have nothing to base the comparison on … this is really not possible to answer this until you re-create your experience in the store. This means finding the original combination that you liked so much. It would be some combination of foundation, mattress, and topper.
If you cannot re-create the combination that felt the way you remember … the most probable reason is that what you felt was more of a subjective perception which can change from day to day, week to week, or time frame to time frame and will be affected by many other factors that have more to do with how you are feeling at a particular time than they do with the mattress itself.
If you can re-create the combination … then this leads to the probability that the combination itself is the main reason behind what you felt and that this feeling can be somewhat duplicated regardless of how your perceptions and circumstances may change over time.
So the first step is to see if you can re-create the baseline combination which felt the way you remember. My guess is the most likely combination would be the PLB latex foundation with the Beautiful over that and the topper over that so this is the place I would begin. Ask the store to set this up for you. If this feels the way you remember, then you have the baseline as a point of reference and you would then know what changes to make to make your combination at home feel the same as in the store.
This combination (whether in the store or its “duplicate” at home) may still not be “just right” once you have slept on it for a while (the adjustment period) but at least you would have a baseline that was the same as your store experience and you could add other layers to it to make it softer yet if that became necessary.
If it does turn out that the “perfect” combination was the mattress and topper on the latex foundation … then you would also have a rather difficult choice to make about whether duplicating this feeling was more important or whether having an adjustable base was more important.
If you decided on the adjustable base even though it wasn’t as soft as the combination you loved in the store … then you could start the process of adding toppers to what you have to make it softer (bearing in mind the risk to your alignment). A possible alternative to this would be to use a softer topper instead of the one that you tested in the store to see if that came closer to what you want when the mattress is on an adjustable base.
So this is the question that you will need to answer for yourself by finding out whether you can re-create your experience in the store and find a reference point from which to make some difficult choices and to adjust the combination you are using at home. This will tell you whether what you felt was more connected to how you were feeling that day or whether it was the combination in the store that you tested. Either way … the answer to this “mystery” (was what you felt more about you or the mattress) is the first step to knowing what the next step after this may be. Making adjustments to what you have now without having a reference point for what you originally felt in the store would be like rolling the dice and “hoping” that any changes would work. The odds of “getting it right” would be against you.
Either way there are always “better” solutions (although none may be perfect) … but it’s important to know first which “problem” you are trying to “fix”.
In terms of your husband sweating on the bed … there are many possible reasons for this and a more temperature regulating mattress protector may help a lot. While latex is more breathable than other foams … the softer a mattress is and the deeper you sink in … the more likely the odds that you will sleep hotter (there is more “insulation” around you when you sink in deeper). Outside of controlling external temperature with an air conditioner (which may cause issues for you) usually a combination of mattress protector, more breathable and natural sheets, less bedding, or temperature regulating materials or fabrics can solve the problem. Synthetics in any of these tend to sleep hotter than natural fibers.
Hope this helps.
Phoenix