Hi JNJJ and welcome to the Mattress Underground 
Sorry to hear that you are having such issues with the Mattress Warehouse. Sounds very frustrating! Though they are correct, in that recommendations for slatted foundations are usually 2 1/2" or less - this is for structural integrity of the support system as a whole, as well as allowing air circulation for the mattresses bottom surface area, you are correct that this should be clearly stated - and the discrepancy in what they told you after the fact is also not great from a customer service standpoint.
Any mattress you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) and your PPP (Posture & alignment, Pressure relief, and Personal preferences). This gives you some guidance as to what your new mattress should provide to give you proper support as well as comfort.
The issue with Serta - and many of the ‘big brands - is that that are very lacking in transparency in their components and construction. Without knowing the specifications - layer height, foam density, details of components (lije gauge & coil count for springs, etc.) - it’s almost impossible to assess how suitable the mattress is for any given sleeper, or the overall likely durability over time. And as they tend to use trademarked foam formulations, this leaves the consumer with their only option to ‘hope for the beset’…which is not a great way to choose a mattress which one hopes will provide comfortable sleep for many years!
As you don’t give any details of your sleep profiles or what your prior mattress history, of what your goals are as far as comfort, there are a few pointers which may help, since it sounds like you are forced to settle with store credit which may help you.
First, if you are considering any latex or latex hybrid bed, Latex Rubber is a known quantity, unlike proprietary memory and polyfoam components. As you can see by comparing the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, Latex is both very breathable and one of the most durable mattress materials you can have…though not everyone enjoys the specific feel of latex! If you go this route, and if Mattress Warehouse has any latex/latex hybrid mattressses, you can be a bit more confident that ‘what you see is what you get’.
If you are looking at foam mattresses or foam hybrids, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
As long as your slats have less than 3" of space between slats, you should have adequate support and ventilation - but always check the ‘fine print’, since as you have seen this is one of the ways mattress retailers use to deny returns/exchanges…another common trick is to say a potentially prematurely wearing foam comfort layer (which can break down due to insufficiently dense foam for the body profiles of those sleeping on it, or have an excess of inexpensive ‘filler foam’, which can leave ‘ridges’ and ‘valleys’ in the foam which can cause muscle aches, pains and even alignment issues) does not exceed their ‘body impression’ guidelines, so that a memory foam layer can be considered ‘not returnable’ even when the mattress proves to be uncomfortable for the consumer.
You may want to consider some other options that they offer, even with their ‘shady practices’ that you’ve experienced so far, since your money is tied up in store credit; it’s quite possible that there is a perfectly good model in the store which will be comfortable, supportive, and durable - you just have to take some time to consider your needs and their various options available. Hopefully, other forum members can also chime in with their experiences with other Mattress Warehouse
mattresses which they have been happy with, to help you in your decision!
~ Basilio