Well, I maxed everything out and let it run for a few hours and my stone rolled over back to zero, so that answers the overflow question. (Filed under: Things to do after you beat the game)
98, but I doubt that I'll top that, for the same reasons others mentioned. Once the reds come out it's really down to the luck of the placement. Question, do the number of zombies on the screen influence how many zombies are added the next round?
I have to agree about my dislike for the auto-aim. For me, the joy is getting bullet penetration and lining up shots to take out the maximum number if undead with minimal ammo expenditure. I've lost that fun in the latest update.