This is crazy talk, lol.
1) Maybe the battery is toast or maybe it just needs a full charge put on it. I'll take your word for it that it's toast.
2) Carmax? You were asking to be reamed. That's as far distanced from responsible seller status as it gets. They treat cars like they are a broom, not something that needs assessed on an individual basis.
3) If they install the cheapest battery, you just accept that because you're too deep in to come out better otherwise. It will still be okay for a "few" years.
4) There is no need to replace it now, just to recognize that being low end, it will need replaced sooner.
5) No, there is no reason why the 12V battery needs to be OEM. In fact the OEM factory battery was probably not high end in the first place, penny penching by the factory to just meet minimum expectations.
6) No, your carmax experience wasn't positive if you consider that you just payed more for their involvement as a middleman, but that does have some value if they handle any problems.
7) Reputation for early failure doesn't mean anything, just that the factory put in a budgetized battery that won't last as long as a premium priced battery would. You can make the same choice, depending on where "western USA" is, you may not have cold enough winters where you need more cca than budget batteries provide, but hot weather also degrades them.
Overall, it seems like you want extra attention because you just bought something, but with no problems from it??
If you want encouragement to go after carmax for more than they gave you, it is a losing proposition. Their profit model is very anti-consumer driven and it's now 100% on you to just assume ownership and proceed as if they are out of the picture.
You are worrying too much. Wait until there is a problem and then deal with that.