When I went, the viewing area was open by 7 or 7:15.
The After Fireworks & Plaza Garden parties share the same area (there's usually plenty of room for both though might vary depending on how crowded both parties are). That of course means while Plaza is gnoshing on desserts, After Fireworks essentially get first dibs on the area. So yes, After Fireworks means you get to stake out a spot and then after you view the show, you go back to gorge on dessert.