Hi suspent.
Welcome to our Mattress Forum. 
It’s always a challenge buying a mattress for ourselves, let alone trying to factor in the support needs and comfort preferences of another person, especially when that person has a very specific set of preferences!
We have had the Sealy Posturpedic mattress come up a few times in the forum. You can read Phoenix’s analysis and review of that mattress here and you may also find this forum topic interesting/useful (especially some of the additional resources it links to as it pertains to selecting a durable mattress).
As you’ve likely read already on the forum, 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 we would generally suggest avoiding all of them completely.
Quality Sleep Shop and Flexus may have options that you may find interesting. This isn’t to sway you one way or the other; like you mentioned, Costco returns tend to be hassle free (I would double check though, with a mattress, as that’s not just something they can take back/re-sell. They may have different policies) so if you and your mother think she’ll like that mattress, you can always give it a go. If you find it’s not.a fit, you can return it and resume your search. While we generally wave people away from these larger brand mattresses, they DO work for certain demographics of people, and we have no way of knowing if your mom is one of them.
I hope this helps!
NikkiTMU