Where BB passed the girls is at the bend between Mears and the bridge. The bend is where where the eyeshot opens up from the trail and extends straight down the trail and down the bridge because the trail is old railroad tracks.
There is no chance he did not see or pass the girls.
I agree that scenario 1 is the most likely scenario, but the problem is the timing by witness BB.
According to Hoosier Harvest Store surveillance camera, BB's SUV passes the camera at 1:46 pm on her way back to the Mears parking area from her bathroom break at the local library. She arrives at 1:47pm. Let's give her a minute to gather her things and get out of the car.
She is there to exercise so my assumption is she is continuously moving and walking. It takes 5 minutes to walk from the Mears parking area to the start of the Monon High Bridge. She arrives at Monon High Bridge and sees Richard Allen on platform 1 about 1:53pm. KG's car is seen driving away from the Mears parking area at 1:49pm by the Hoosier Harvest Store surveillance camera. This means Abby and Libby were probably dropped off shortly after BB began her walk to the Monon High Bridge at about 1:48pm. In my opinion, I think BB probably saw Abby and Libby closer to the start of the Monon High Bridge, but who can guess how fast or slow Abby and Libby were walking.
According to BB, after she sees Richard Allen at 1:53pm, she goes back down the 501 trail and on the way back to the Mears parking area about halfway walking back passes Abby and Libby. This would put her sighting of Abby and Libby at about 1:55pm. Abby and Libby arrive at the start of the Monon High Bridge at around 1:57-1:58pm. They walked slow. The picture of Abigail Williams at platform 3 looking back to the start of the bridge with no one in the background is at about 2:07pm. There is an earlier picture about 2:05pm of the entire length of the bridge looking towards the southeast end of the bridge.
Where was Richard Allen during those 10 minutes? Where was Abigail Williams and Liberty German during those 10 minutes?
It seems like there can be 4 possibilities:
1. Abby and Libby arrive at the start of the Monon High Bridge around 1:57pm. They sit and stare at Richard Allen for the next 5 minutes on platform 1. He eventually gets tired of them staring at him and gets off the Monon High Bridge to walk back. This delays Abby and Libby getting on the bridge.
2. Richard Allen gets off the Monon High Bridge platform 1 shortly after he is seen by BB, and starts walking back towards the Mears parking lot. He passes Abby and Libby on the 501 trail, but for whatever reason decides to just stop in his tracks and think about what to do next for 10 minutes so this is why he is not seen in the 2:07pm picture of Abigail Williams.
3. Abby and Libby do not care that Richard Allen is on platform 1 when they arrive at the Monon High Bridge. They start to walk across the Monon High Bridge while he is on platform 1 looking at the fish below. Abby and Libby pass him on the Monon High Bridge before Liberty German takes the 2:05pm photo of the entire length of the bridge once they are beyond platform 1 where Richard Allen is standing. But this does not make sense because no one is in the 2:07pm photo of Abigail Williams which is a photo taken back towards the start of the bridge. Would he really get off the bridge at that point since the girls are already beyond him and moving away only to get off the bridge to come back later?
4. Abby and Libby do not arrive at the start of the Monon High Bridge until around 2:05pm because they do not start walking to the Monon High Bridge until 2pm, but if BB is continuously moving she should have started her walk down the 505 trail where the overlook bench is located at around 1:58pm when she gets back to the Mears parking area. The timing is tight for Richard Allen to have sat on the bench shortly after BB's starts her walk down the 505 trail and Abby and Libby start their walk to the start of the Monon High Bridge. This might explain the missing 10 minutes.
None of this takes into consideration the over 60 confessions, unspent toolmark analysis matching Richard Allen's gun, or the white van confession that only the killer would know. But this is why I think it is important to verify when Richard Allen left the Monon High Bridge trails that day. If his original confession is correct, I think his car should have been spotted on some surveillance camera around town driving after 3:30pm. I think that would be the easiest way to verify his story instead of trying to guess how fast or slow people walk on the Monon High Bridge trails.