First, to a comment that was made in that thread about Optima's, these batteries are excellent batteries and been using them in all my cars for over 6+ years with little problems. Optimas can go bad if the car does a lot of sitting as I've found out and afted many conversations with their techs. Optimas need to be used and often for them to work right, the parasitic(car off) drain is what can kill these and really any battery, so you need to find out your parasitic draw outlined in my thread below. Make sure ALL lights are off, doors/trunk closed and wait 5 minutes...it should be less than 50mA or 0.050A(mine is stock and 32mA). Any new battery can go bad from the very start if it was manufacturer defect, usually a bad cell. New batteries can also be low on the shelves but know that a free load test should always be done to know for sure. From the sounds of it, you have a digital volt-ohm meter with hopefully 10A DC capability(even the $5 meter at Harbor Freight does). Something as simple as engine grounding or a connection can cause these issues or high drain as mentioned. This can be easily found and fixed.
Usually an alternator bench test is the best check method and is correct because it places a heavier load on the alt verses sitting in park idling(as well as checks AC ripple and possibly diode leakage), but i don't always trust the store employees tests so if it said it was ok/bad i get a second opinion or test. When testing an alt with a DVOM you want to test what you can and what the bench test mainly does(except for load testing) such as high AC ripple, diode leak(causing battery to die while off), and poor connections that cause high resistance. A DC clamp inductance meter can help as written in the link. Just an fyi, if it turns out to be a brush or regulator issue you can obtain them seperatly. Got brushes for my Denso that's still in the car for $10 off ebay.
Anyways, If your battery symbol didn't come on your cluster it is possible the alt is good. That symbol does not always nessesarily mean the battery is dead either, it means charging/running voltage is too low(below 13.2vdv). Usually at that point it is the alt if it's illuminated. Which by then if it was a bad battery it would take out the alt by then. Most people don't realize a healthy battery is needed to buffer the voltage spikes from an alt and is what it regulates, otherwise it can easily damage the $600 alt.
There's a chance your alt was good but finally got taken out by low voltage(bad battery), so i would get a second opinion if you can at NAPA if they're around ya, they seem to have the better testers.
You will want to do these tests if you have a DVOM. These tests will show you what's wrong so you don't waste more money, that's why i wrote it up.
Post back your parasitic draw and other values and I'm sure we can figure this out for sure. Sorry for the book, just wanted to make sure i covered everything discussed.