You won't need water if you use wet burlap. Season, inject and stuff the pig, wrap in burlap and hit it with the garden hose to wet it down. I've wrapped pigs in foil and they still come out moist. The underground pit makes meat nice and moist.
I'd up the hot coal amount, 1" doesn't sound like enough heat to make the hot rocks last long. Not sure what you plan on using but hardwood coal is going to give you a longer cook. Charcoal will not work well.
A 30lb pig isn't very big but when I cook pork butts, (no rocks) they still take at least 8 hours.
If you butterfly the pig it will cook fast. If not, I'd plan on at least 8 hours, give yourself time to let the pig rest too.
When cooking a large hog (without rocks) I make the hot coal bed about 1 ft deep so it will last for at least 12 hours.
It's hard to pin point a time for a 30lber.
Good luck with it!!