A few comments:
1. The bridge is more walking than 'hiking'. You walk down a shortish, flat, maintained trail and then you walk out on the bridge. You return by backtracking. It wouldn't be a satisfying hike to a real hiker. If you're looking for a greater challenge, I suppose you could walk the 505 steps down to the creek in the Mary Gerard Nature Preserve, which begins at the same W300N trailhead.
2. I think the trail was upgraded after the opening of the 25/Hoosier Heartland Highway in 2013. The benches, signage, bridge over the 25 are all fairly recent. I know the bridge has been a 'trail' since 1987 or so but was probably even more low-key until the upgrades.
Yes, he did. One thing that visiting the Monon Bridge makes clear is that BG is a local or has spent enough time in the area to be considered local.
The terrain is difficult. Dense woods even in winter. It is very difficult to orient yourself to the winding river and to navigate the roads around it. The access roads close to the bridge are almost impossible to identify unless you hike in and walk them on foot.
And if you're going to spend a lot of time scouting the area, chances that you'll be seen by someone are very good. The trail is close to the road on the north side. The trailhead where the girls were dropped off is right in front of a Mears house and people are often working outside there. On the south side, the only access is by passing a succession of close-to-the-road residences on 625W. There is no turnaround except by either backing up the road or into someone's driveway.
The cemetery, however, dips down at the back near Ron Logan's pasture gate and you can park a vehicle there without being seen from the road. But there's no clear trail down to the river from there, and it's not easy to get to the bridge. From the looks of it, it seems very unlikely that anyone could drag two adult-size bodies through the dense woods (even in winter) down to the body recovery area -- at least without it being very, very obvious that someone did this.
This was not in any way a casual, opportunistic crime committed by a passerby or transient. It required knowledge of the area and -- this is so important -- loads of confidence in that knowledge.