Why you have two garboard drains, I think is because it's an inboard with a pretty flat stern. Usually on V bottom boats you have a single drain at the lowest point of the bilge in the back. But the Centurion is pretty flat back there as a competition boat. Plus, as an inboard you have your exhaust coming out the back in the middle. Not certain that's the reason, but it's my best guess.
If the hole in front of the engine is going down through the hull out of the boat, I'm not sure ... don't recall that one. If it's going forward from the engine into the boat, that's to allow water from the bow to drain back to the bilge.