IMO, LISK knew that stretch of road well, and traveled it often. It's possible after the placement of the first body/selection spot he took note of something topographically, maybe a taller pine tree in the bramble, dip in the bramble, some sort of visual cue. Perhaps the first body was placed in that way, and then he just revisited the first spot during the next dump, and used visual cues to pick out spots for the others. Just thinking out loud.
As for amount of killers, two, or more, I tend to lean toward the thought GB4 are one perp, younger in age. The other bodies could be the work of the same killer, but I think it's more likely to be an older perp, with different style. Whether they knew each other for sure, or crossed paths, other than where they placed their victims, I think it's difficult to ascertain.
One thing I have thought about is that GB4 stopped placing in that area when the other bodies started showing up, other than GB4. That may have alarmed him that it was not a "special" place, just for him, and that someone else was using it. I think if the killer of the GB4 has continued, there could be other parks or beaches near the south shore he may have switched to, to preserve a "new" garden of his making.