SMP,
I cannot imagine the heartbreak your dear family has gone through. My prayers are with you all tonight.
I think the car into the water theory might just be the answer. I've posted this in another forum, but for your benefit, I'll repeat.
My cousin's husband accidentally drove off a boat ramp while lost in the middle of the night and drowned. The ONLY reason he was found is that a woman was letting her dogs out nearby and heard the splash. He was totally and completely lost, and the boat ramp looks for all the world like a two-lane country road.
My son-in-law's father fishes every Thursday in the St. Joseph River for decades. He has launched his boat in exactly the same spot hundreds if not thousands of times. Several years ago, the police recovered a long-submerged car and driver- from directly under John's launch spot.
I for one would like to see a volunteer effort among the country's fishermen to explore with sonar every single body of water in our country for missing cars and people.
Not sure if this helps, just wanted you to know I'm thinking of you and praying for you today.
I truly think Brian is in the water or possibly off road but in that stretch from party to home... I think they should hang new flyers and have people search... it's time for answers for this family...