Let me start with an apology for how long this post is and say that I think this group of people is outstanding in both what they do and they way they go about it. Thank God for people like you!
Here is a theory I havent seen discussed and its based on two child molesters I know of in real life. The men I know of, first the father, molested one of his daughters for years without being caught. He divorced his wife, married another woman who had to girls from a previous marriage and molested both of them (not reported) to authorities. The wife left him and he again married, another woman with a daughter. He molested that daughter and tried to get this girls cousin and her to have sexual contact with each other in front of him. Once again, this was kept secret by the family and the wife stayed and the abuser joined the wifes church, confessed to the minister that he had impure thoughts and was forgiven (by the minister) and a prayed for by the minister. He did not die until years later and the wife eventually left him.
This mans son married a woman with one daughter from a previous marriage and molested her daughter off and on for a period of years, beginning about age seven or eight. By the time she was sixteen, he got braver and tried to have her call her friend into the bedroom while he made the girl have sexual contact with him. This girl (the friend) did not tell her parents but the mother overheard her phone conversation with another school friend discussing this and laughing nervously. He mother called other mother, told her and advised she was also calling the police. In that case the mother also reported it, moved with-in a few days time and this man went to prison for not nearly long enough. This was before the mandatory registry laws, and though someone else had posted that the law went retroactive to some fifty years, my experience does not find that to be so.
Having said all of that, here is my loose theory which is MOO. A friend of the families has been abusing his child, grandchild, niece/nephew and had been grooming one or both E and L, having become braver. This person, since he/she was known to the family, could have enticed the girls away from their bikes on the day they went missing with any number of things, including the child he had been molesting. There could have been a vehicle parked in the spot that has been discussed as a possible parking spot. With the intent to see what he could get away with, never expecting this to go viral with FBI, CSI and all, he/she could have panicked and had to dispose of the girls.
Second possible scenario assuming all of the previous but knowing his victim had told one or both of the girls what he/she had been doing, he/she planned to take them for a ride and have a very scary talk with them about what could happen if they told anyone what his victim had divulged to him/her. Once again, things took off very fast in trying to locate the girls and he/she had to silence them permanently.
This person could have known the families schedules for that day just by casual conversation with a family member or, more likely, arranged to meet them by the lake or meet his victim by the lake that day.
The purse being on the opposite side of the fence I havent figured out unless this person or his victim knew E had a cell phone that she carried and grabbed her purse and threw it over the fence.
What do you all think?