Hi funnygarcia,
Post #2 here has more about exchanging a mattress in a mainstream store where there are no great options available to exchange for although I’m guessing you may have already read it since your idea about a “firm mattress + topper” is one of the suggestions mentioned there.
Unfortunately this is part of the “trap” of most exchange policies where you can only choose the same or a higher priced mattress which may not be the best choice for you because higher prices can often mean even more lower quality polyfoam that can soften or break down too quickly.
This isn’t “too bad” because the marvelux is generally a little higher quality/density foam and the apex coils are pocketed coils so this would make a reasonable “base” layer for a topper.
This one isn’t bad either but it has more “questionable” foam than the Sealy.
Assuming that you can confirm that the “hybrid latex” is actually latex, this one only has 1.5" of lower quality/density material and the latex comfort layers would both be durable materials (again assuming that it’s actually latex) so this one could also be a little less risky. It’s also a little softer than the other two (and they rate it the same as the mattress you are exchanging) so you may only need a thinner topper (or perhaps no topper at all) for it to work for you. Of course this mattress may also turn out to be too “soft and squishy” for you regardless of the quality/durability of the materials and the support core is also in a little softer range (28 ILD) and if that’s the case then it’s much more difficult to “fix” a mattress that is too soft than one that is too firm.
I don’t see any reason why the foundation for your mattress wouldn’t work for any of the other options you are looking at.
Phoenix