Hi PaidNewGotUsed,
Welcome to the Mattress Forum! 
I’m sorry you’re having issues with your new Serta mattress. I don’t have any advice on how to determine if you’ve received a mattress that was a “comfort exchange”, except the obvious signs of wear and also looking at the production date on the law tag and seeing if it is quite old as a potential indicator. The mattress you purchase uses quite a few layers of very plush materials in lower density ranges, and it certainly would be possible for this to impress quite a bit in early use and not be an indicator that is was used… Also, in the assembly, it is possible for the top quilt panel to be a bit too large or not be joined properly (tightly enough) to the pillowtop gusset, and this can also contribute to this problem. At a distance, this would be a few things that initially come to mind. I would also make sure that whatever foundations you’re using (if they’re not new ones that came with your mattress) are firm and non-flexing, and supported by a proper bed frame.
As I think you’re already aware, the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
However, in situations like yours when your credit lies with a store selling these types of items, my general recommendation is to choose a mattress that is on the firmer end of the spectrum and uses the least amount of these lower-density foams, and your idea of using some sort of a high-quality topper is a good one, as this can help to best extend the life of a mattress that you have available to you with your store credit. I’m not familiar with the complete lineup at the Brick, but I agree that you are helping set yourself up for a better comfort life for your new mattress with your idea of a firmer mattress with a topper.
One note: a “firmer” poly foam doesn’t necessarily mean that it is higher in density. The firmness is the IFD/ILD, and this is independent from the density, which is the best correlation to durability. It would be very unlikely that you’ll find any polyfoam in the products you’re considering that will use higher-density foams, which is why I recommend to minimize the foams as much as possible within the mattresses you’re looking at.
Phoenix