Hi wacomme,
Some of the answers to your questions are a little more complex than you may realize and while a topper (latex or otherwise) on an upside down Tempurpedic may work for you … it also may not depending on many variables.
The first rule of mattresses and sleeping systems (including mattress/topper combinations) is that only your own personal testing or sleeping experience will tell you for certain whether it is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and there are too many unknowns and variables involved for anyone to be able to predict for certain whether a mattress or mattress/topper combination will be a good match for you based on specs (either yours or a mattress) or "theory at a distance (see mattress firmness/comfort levels in post #2 here).
If you can’t test a mattress/topper in person then it’s always a good idea to make sure that you have good options available after a purchase so you can make any firmness changes that may be necessary for either you or your wife if your choice doesn’t turn out as well as you hoped for because choosing a topper that is a good match for both of you on a specific mattress (the mattress under a topper will affect which topper thickness and firmness would be best) can sometimes be as uncertain as choosing a suitable mattress in the first place (see post #2 here). Of course you would already have the “base mattress” to go under the topper so the tradeoff for the additional risk would be the lower cost compared to buying a completely new mattress.
It also possible that your needs and preferences are different enough that a topper (or a mattress) that works well for one of you may not work nearly as well for the other. There is some information in the first part of post #2 here that may be helpful for couples that have very different body types or sleeping positions and have very different needs or preferences but of course your own testing (with both of you on the mattress) will always be the most reliable way to know whether any specific mattress is a good match for both of you in terms of PPP.
If your tempurpedic turned upside down has a firm and even sleeping surface and there are no soft spots or sagging (either visible or virtual) that are “coming through” from the top layers that are now on the bottom (particularly under the heavier areas of your body) then it could be a suitable choice for a topper and this could be a good way to extend the life of your mattress for a few years … again assuming that the mattress/topper combination is a good match for both of you in terms of PPP. Adding a topper to an upside down mattress is one of the suggestions in post #4 here that can sometimes help a mattress that is too soft or is sagging when there are no better options available or inside the budget.
This could be a risky assumption because whether a mattress is a good match for you depends on all the layers in a mattress or sleeping system and how they all interact together with your body type and sleeping positions … not just on the top layer. The type of material is not nearly as important in terms of PPP as the specific design of the mattress or sleeping system. The type of material is more of a preference choice than a “better/worse” choice and the design of a mattress or sleeping system and the firmness of all the layers is a much more important part of whether a mattress is a good “match” for you than the type of material that you may prefer. Post #2 here and post #4 here has more information about primary or “deep” support and secondary or “surface” support and their relationship to each other and to firmness and pressure relief and the “roles” of different layers in a mattress that may be helpful in clarifying how different layering combinations work better for some people and body types than for others.
A latex topper on a firm mattress certainly wouldn’t be the same as an all latex mattress and would be closer to a latex/polyfoam hybrid. While the upper layers of a sleeping system will generally have the biggest effect on how a mattress feels when you first lie on it (depending on weight and sleeping position) … all the layers of a mattress will affect all the other layers and components to different degrees. There is more about how a latex/polyfoam hybrid compares to an all latex mattress in post #2 here. I would also keep in mind that there are hundreds of different latex/polyfoam hybrid combinations (or mattress/topper combinations) that will each be different from each other as well and that some may be a suitable choice and others may not be.
If you do decide to go in the direction of buying a topper to use on your upside down Tempurpedic then post #2 here and the topper guidelines it links to has more information that can help you choose the type, thickness, and firmness for a topper that has the best chance of success and also links to some of the better online topper sources I’m aware of that would be well worth considering both in terms of price and because some of them also have good return or exchange options that would lower the risks involved in choosing the most suitable topper.
A topper cover is also a good idea for any latex topper to protect the latex from handling (latex can tear easily) and from oxidation or from breaking down prematurely from exposure to substances that can harm it over the longer term (see post #12 here and the posts it links to).
Phoenix