THANKS to all who provided useful advice or outside-the-box ideas! Different day, different employee, different result. I brought my old battery back to Walmart again to see if 2/3 employees would honor the warranty, or if 2/3 would not honor the warranty. It turns out 2/3 employees would honor the warranty. Mission accomplished.
For all those asking, I don’t know who installed the battery, but it wasn’t me or the quality mechanic who fixed the old (now exchanged) battery to ok albeit something like 90% working-power condition (due to lack of cylindrical shape of battery terminal). And to anyone who doesn’t know, it reflects poorly on you when you blindly judge/insult people who have zero capability to defend themselves. The mechanic who fixed my car came as highly recommended from a local electrician, and he let me watch over his shoulder and taught me everything I could possibly need to know about the future of my car’s battery situation. Sometimes it’s hard for me to trust mechanics. This one is the best I’ve had in my life. 