To answer your first question, destroying the machines right before the time runs out might not always be the best strategy as the energy generation after 30 second mark is next to none. But I have to agree this idea seemed more fun as an idea than it was in reality. Maybe it would have been more fun if the machine prices, energy consumptions etc were more balanced.
To be honest I don't exactly know what's the deal with the central machine, it was drawn by Ghostsilver and I'm not exactly sure, what his plan with it was, he probably has to comment on it himself.