Pathfinder Battles Case Subscriber
I'm running this as well and here's my understandings on your questions
Jumping allows them to make an attempt to move rubble that same round, running down will take the entire round and a second floor isn't needed.
You don't necessarily need stats for all the npcs and I would place then randomly on the map since they're all practicing different parts. It doesn't matter if any die (though they probably will from an attack), the pc's fail that part of the secondary if anyone gets injured.
If it drags an NPC down there, I'd assume it retreats unless attacked. If a pc is dragged under, continue having them act while they can. I wouldn't allow persuit under the tunnels without very very good reason, especially this weekend when tone is of the essence. I'd have them show up down in what I assume is a sand area at the base of the stage, attack whatever is closest.
Hopefully this helps