Hi NaiveSkeptic,
Unfortunately I don’t think they are much different from most of the chain stores that dominate the markets in most of the cities around Canada and the US and as you can see in the guidelines here … I would extend this to chain stores in general which usually won’t disclose the quality of the materials in the mattresses they sell.
As you probably know … all foam will go through an initial softening and break in period but lower quality foams will generally do this more than higher quality/density foams and in some cases even this initial softening can put you over your threshold and lead to the loss of comfort and support and the back pain that you experienced (although this has nothing to do with the quality or value of the mattress … it’s just a mattress choice that wasn’t suitable for your needs and preferences).
As SleepySam mentioned … the good news is that you have a mattress that is too firm rather than a mattress that is too soft which is easier to “fix” by adding a topper.
Unless you have tested a specific mattress/topper combination though … a topper can be as difficult to choose by “theory” as a mattress because a topper will interact with the layers in your mattress in sometimes unpredictable ways so what would work well on one mattress may not work as well on another. The most accurate way would be to test a topper on a very similar or identical mattress so you can see how they interact. In most cases of course this isn’t possible so at the very least toppers that you test in person should be tested on a mattress that is as close to yours as possible. In the case of memory foam … a topper will also affect how the memory foam responds because memory foam is sensitive to heat, humidity, time compressed, and pressure, all of which will change when you add a topper. ILD in memory foam is not particularly meaningful and changes with conditions (although the Optimum memory foam is not 24 ILD to my knowledge which would be very unusual for any memory foam).
The best way I know to choose a topper is to use the guidelines in post #2 here (and some of the more specific information in the posts it links to as well). This will give you a reasonably effective way to assess your own mattress and decide on how much more pressure relief you need and choose a topper accordingly. The best choice is usually “just enough” to provide the extra pressure relief you need in your most pressure prone sleeping positions so it will have less effect on your alignment in other positions (such as on your stomach).
I would also be very cautious about using very low ILD latex, especially in thicker layers, and while the PLB toppers use Talalay latex which is a good quality material … they are also very costly compared to the other sources of the same materials that you have access to in the components and toppers post.
Brooklyn Bedding also allows returns or exchanges on their latex toppers but I don’t think they ship to Canada.
SleepEz also has toppers with wool in the covers here and they do ship to Canada but are not returnable to my knowledge.
Some of the online options available for Canada which may carry toppers are also listed in post #21 here.
Hope this and some of the “reading” I’ve linked helps you make a good choice 
Phoenix