I agree with all this. I can make myself see a dog there, but I don't think it is. Like you said, the use of a real and/or imaginary dog to entice young girls wouldn't be an unrealistic thought, at all, but in this case, BG didn't need anything to lure/trick/or otherwise deceive them once at the south end of the bridge. There really was nowhere for the girls to go, especially if he had a weapon. The family has described the audio as basically happening when the man met them at the very end of the bridge, and nothing much more. No "look at my dog," "hey, I'm a park worker and you're trespassing," or "my buddy needs our help at the bottom of the hill," or anything else like that. LE has said there wasn't much more to the audio, so I respect that for now.
I'm not dismissing the theories put forth on here, because none of us know really anything. I've had my share of unpopular ideas. We're all just guessing, but the interpretation I get from the few details we've been given, was that BG reached them at the south end of the bridge and immediately took control, per LE via "intimidation and manipulation," which to me suggests a weapon. That really just makes the most sense to me, personally.
