I have looked at the photos of the road and property that you have posted several times now and the same thoughts keep coming to me.
While the lay of the land in the back is lovely, it would be very hard to traverse, especially while carrying a child. I also believe that in the short amount of time that Annasmom realized that Anna was missing they would have been able to see someone on the hills of the property.
However, the road running in the front of the house appears to be fairly narrow with a lot of trees on either side, except where houses are. There would be a lot of places to hide to wait for an opportunity and to hide to wait to be picked up.
I know many may scoff, but I truly believe that the Georges planned Anna's abduction, observed the routine of the household, had someone else approach Anna while she was playing alone (she probably thought this was another friend of her Mom or Dad Joe), the perpetrator used a drug to disable her for the time being and waited in the woods with Anna to be picked up, by whomever was watching the scenario unfold. The Georges then placed her for adoption with another couple through a not so legit attorney or placed her with a couple in one of Brody's wacky groups of interest.
JMHO.