on the diagrams Walt.
Tyler correct me if I'm wrong, but I believe you indicated at around 12am all seemed OK. I assume the boat was in the water for quite some time prior. It wasn't until around 4:45am that the water was noticed. This may have been some time after you and others settled down for the night, possibly adding additional weight to the boat.
I got the impression you are uncertain as to whether or not the ballast tank drain plug was tight. The pics you showed us of the rubber gasket looks like even if you tighten the valve and the ballast vent hole plug was not tight, with people on board, gear etc., water could potentially get into the bilge, the more water, the more weight and the faster the water would come in,
if the vent plug and valve were not tight.
I totally agree and understand if the valve is wide open and the vent plug is tight, no water will come it. (ensure never to sail with it open, because when the boat heels water will escape from the ballast tank)
I also got the impression after the boat was back at the dock, people off, no more water intrusion, maybe because you bailed out all the water from the bilge, people off, now the vent hole was above the water line.
My thought is simple, fix what you see is wrong (rubber gasket) maybe get a new ballast tank vent plug, test it out, see what happens! I hope it's not a leak at the transom.
Maybe you can post some pics of the transom. The one's I've seen that leak it looked very obvious in pics there was a problem.