Two of my balls have had the thumb plugged probably 4 times and moved within an inch of each other, plus the fingers moved slightly.  They're symmetric, but even though all the plug is in the same arrea it seems as if one of them has gotten a little more backened.  I think it's because the core has lost of of its symmetry, even more so than a first drill because plug can probably only add a fraction of the density back.  As for performance loss, I think that would be just due to the amount of use that's been put on it like you said.

As for the warranty still being good if you're the second owner of it, I think you're right that it'd depend on the manufacturer, but for the most part I can't see the company replacing it.  Doesn't the ball have to be less than a year old for the warranty to be good?  The reason I ask is because if you get a used ball with 15 or less games on it, there's still a chance the ball could crack since it hasn't been used that much(if it's defective?).  Although, besides drastic temperature changes, balls will usually crack very quickly(5 games or less) if there's something wrong, right?