Hi tempured mad,
A successful warranty claim would only allow you replace the mattress but not receive a refund. You could sell the new replacement mattress on Craigs list or Ebay or through social media but of course you would take a substantial loss compared to the full price of the mattress.
The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality 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 (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).
It sounds like a reasonable plan to me but of course the only way to know for certain how any mattress/topper combination will feel to you and whether it will be a good “match” for you in terms of comfort, firmness, and PPP will be based on your own careful testing or your own sleeping experience on the mattress/topper combination.
Phoenix