Justthinkin': I thought a lot about Rusty's theory that Rachel and Renee ran away and gave Julie to someone else and somehow Julie died and so on. To me there are several problems:
1. Renee just received and accepted a promise ring that very morning.
2. If Rusty's idea that Debra is involved was correct, I could understand Rachel remaining loyal to her sister because they were so very close, but the situation is very different for Renee. I would think when Renee realized she could NEVER see her parents, grandparents, cousins, aunts, uncles, friends and new boyfriend again, she would resent the situation and she and Rachel's friendship would have broken down as they matured. At some point I think Renee would have returned and told police the truth. If she had participated in any way - such as hiding the body, she was underage - only 14 at the time, so I doubt she would be in much trouble. An adult would realize that. Between Debra passing two polygraphs and the lack of logic to Rusty's theory, I just don't see how this would work.
3. Asking multiple people to go shopping with them that day - these are not the actions of two people who are going to run away.
4. Renee's Christmas gift for Rachel's stepson - why bother or spend the money if you are running away?
5. Picking up layaway - why bother or spend the money if you are running away?
So for the above reasons, I feel certain that these girls were abducted by a stranger or someone they knew but should not have trusted. But I would be just as interested in reading your theory as to how this might work if the girls were runaways! You may well have thought of some stuff I haven't and with a cold case like this, the more thought we put into it, the more likely we are to be able to help these families somehow.
As to the airport - I think there is a lot of development going on in that area and many times that is how bodies are found. It is an interesting idea and I really enjoyed the article you linked here! That airport was the first one I ever flew into, btw - I was an infant and my parents lived in California at the time and my mom couldn't wait to show me off to her family, so she flew cross country with me to that very airport where her dad picked us up!