I have posted my theory and the reasons behind it many times. You can find it here somewhere. I don't think a stranger was involved. Not sure exactly who was sent to grab HaLeigh, but I would say it was family/friends to Crystal, and maybe it could have been Chad. We don't know.
You are right, we don't know, but I highly suspect no one was sent in to grab Haleigh...Reason being... Haleigh was not abducted by anyone...
And like you, I have posted my theory many many times, and it can be found somewhere around here also.....JMO