If you're at the point where you're hiring an outsider to investigate, you effectively have an obligation to let them do their job. That means staying out of the way, because anything you do poisons their inquiry.
If you weren't hiring an outsider and were investigating internally, you still wouldn't talk about it in a fucking meeting until you know what happened. You talk to each person individually to get their account. "Interrogating" witnesses in a group both violates the privacy of the (alleged) victim and lowers the quality of their recollection of events because they get shaded by everyone else.
The fact that people weren't aware of the appropriate method of elevating complaints is bad (though not as unusual as it should be). The rest is pretty standard.
Baseball throw it at the backboard with your best fastball every time you touch the ball.
Or even better, do the double play turn where you're throwing it without any kind of grab first.