As you know, a lot of water can come down the inside of the mast into the bilge. Gallons of it.
Seems to me that small leaks from deck fittings and such wouldn't explain big volume of water. (The bilge water is confined to the TAFG sections connected by limber holes, right?)
Sometimes in a deluge the cockpit lazarette drain channels can be overwhelmed, but that would leave evidence of wet in that locker.
One source of massive water might be the chain locker pan itself. If split, it would pour unseen directly into the bow compartments and then drain through limber holes to the main bilge. Or the drain hose might have fallen off the tube in the stem, and be draining directly into the boat instead of out through the stem.
If it were me, I'd find a dock hose and blast every fitting or suspected source of intake. Even hidden leaks, as from the deck fittings or disconnected chain locker hose, would have to reveal themselves under such a fire-hose attack.