Okay, I read the basic rundown and quite a bit more of your theory. I think you're probably the closest to right that I've seen. But, there is at least one fact that is wrong in your scenario. The shop was not TT's it was Rachel's father's. Given, he could not work at that time due to cancer. But, I feel his direct involvement and relationship to the family is important.
I'm her cousin, I was born almost directly after she disappeared. I've grown up with her missing posters every day of my life. The stories of her are only really being told now that I'm grown.
That being said, I know this family intimately. I know we can remain silent for decades about things we don't want public. In my opinion, someone in my family knows something. TT couldn't have sunken a car without the records showing it gone from the shop. But, that could be covered up. He couldn't have been violent frequently with Rachel without a family member knowing about it, there was one that lived with them. It just seems like he would have to have an extra person to help work out these details. To this day, if you mention this case and try to talk about it to almost anyone in my family, they clam up quick. With very few exceptions, Rusty being one of those.
As for Renee and Julie, I do feel like they were at the wrong place at the wrong time. And, I feel for their families and them every single day. I've thought through every scenario, and had read up on this page a few years ago before you started posting. And, for them any way it went was horrible. I know Rusty feels for them too, but I can understand that tunnel vision. I can also understand how there is an extra guilt knowing that had they not been with Rachel that day, their families would be enjoying their loved ones today.