On a day with OUAT before HEA (basically when HEA is at the same time as park closing), anyone doing the plaza dessert party has to decide to either leave the party early and watch from the plaza, or skip the projection show and stay in the dessert party. On a day with OUAT after HEA (the normal schedule), you can stay at the dessert party up until the fireworks then just relax in the plaza until the projection show. Yes, people can come into the plaza, but most won't and enough people leave that it still remains uncrowded.