I could be imagining things, but didn't they have to stop tape once to switch to a new tape since they didn't anticipate going for so long?
I agree that the probability of a dead audience was low. I was in the audience for the Game Show Marathon episode of Press Your Luck -- when there were one or two spins left in that very close final round, they stopped tape to make sure Ricki knew what to say, and the audience was so into the game, they objected loudly. I can only imagine the energy in the audience during Larson's run.
As for switching the slides around, the problem is that they would still need one square to cycle through the $3,000; $4,000; and $5,000 + one spin spaces (because otherwise, where would Big Bucks go to?). And since at the time, the staff didn't know exactly how Larson was consistently hitting the spin spaces, there would have been the possibility in their minds that even if they moved those slides to, say, the third square, he might have been able to start hitting that square consistently. And, yeah, whether Standards and Practices would have allowed it anyway.