Re Ball Stacking, if players could switch balls between rounds they would constantly be doing so and it would slow down the game.
Nice to see, BTW, that the options to purchase additional balls of the same type during a round have been greatly expanded, with:
1) a reminder when you get down to 1 ball,
2) a reminder when 1 hit is left on the ball,
3) an explicit choice to continue with starter balls or buy fresh balls when you run out of hits, and
4) if you do choose to continue with starter balls, an additional chance later in the game to buy more of the original balls you ran out of!
All of this is in the spirit of the WGT one-ball variation and is a total win-win.