Tom
I don't know where you have read reports about drivers cutting the queue - I have never seen this happen on Croatian ferries and nor have I read any such comments.
Everything is very organised and clear. Drive down the hill and past the ticket booth and boat jetty. There is only one place to drive as it's a dead end road. The car queue loops back round towards the dock.
Commercial vehicles are given priority, but other than this, cars are taken onboard in the order that they join the queue. The only exception is if they are towing a boat trailer or caravan, when they may have to wait for the next boat if they are nearly the last on and space is tight.
Drvenik is only a small place, but there are a few cafés where you could have a drink or meal whilst you are waiting. In early September last year, an hour in advance was plenty of time to ensure I got on the first boat. You are aiming for a morning sailing, so the worst that can happen is that you have to wait for the next boat. I would be more concerned if I was heading off the island for a flight or if you were on the last boat of the day.