I'm not really too knowledgeable about diseases. You're getting great advice from the right people on your thread.
In my opinion, Betta (or any fish) don't really "adapt" to high pH and hardness, they merely put up with it. They spend extra energy on osmoregulation, keeping their internal pH and parameters in balance with their environment.
While Walmart fish are not inherently weak or inferior, the type of mass-produced (fishy-mill) fish they buy skews the odds toward a weaker fish.