My guess is that LT wanted to leave home at 16, presumably due to s.a. from a relative or family friend. DT, from that point on, was her rescuer and lifelong protector. Although they both probably wanted a large family, and LT maybe has an overly optimistic view of having a young baby, she doesn't seem to have been a capable parent, probably lost interest as the babies grew older and it might have been a cross between children who aren't really interacted with properly but also expected to keep quiet all the time, and I think that with her focus always on a baby that she probably couldn't 'control' the children very well, especially when DT was at work -- though I would question how much of a father he ever was when he was home from work.
So I imagine a scenario where LT has reached the end of her tether, they've already made the decision to keep the children inside (to protect them from the outside world with its dangers of 'wordly' things? and also probably in part to hide the earlier neglect and protect themselves from outside interference in how they chose to live?) but at some point the older children needing (in their minds) to be kept quiet, kept from raiding the fridge, I envisage LT breaking down and DT seeing this like an engineering problem...solution is to bind the most lively children to their beds, in part as punishment for 'bad behavior' and in part as a fix to a perceived problem. That may have led to locking the children in their rooms and not even bothering to unlock the door to feed the children or let them use the bathroom.
With 15 people in the home I guess hot water might have become an issue and so the earliest answer might have been to ban daily showers. I would think that the restrictions on washing only their hands and not past the wrist would have started as a water-saving measure? Possibly due to LT's own childhood, maybe she associated controlling children with parenting them and loving them? Parents will often tell a child that rules and lack of choices for a child is down to the parent loving them, but in a very strict household, and where there might also be abuse of a child, that lesson will sink in in a different way, and maybe be understood in a twisted way compared to how we'd normally think of it.
I think whoever said that the religious aspect of this is more about the 'cult of LT and DT' has it spot on. I think they've both likely grown up with very strict religious backgrounds, and maybe LT in particular was trying in part to keep the things that she perceived as good, but also to rebel from it and insert the things she felt were good...such of the fantasy of Disney and stories of young women with rubbish lives meeting their prince charming and living happily ever after. Sweet stories that don't really touch on adult topics. Safe stories. So I think they've borrowed aspects of organized religion --the Bible, letting God choose the size of one's family, conservative clothing, etc -- and then added in their own things that are just personal happy things for them, like Disney and Elvis, and it didn't matter to them whether those things would be religiously sanctioned, and maybe if they're not religiously sanctioned it's even more important as it's a part of the rebellion and the building of their own 'cultic regime'.
In the 70s I was a bridesmaid and wore a similar dress to those plaid ones. Mine was white in a satiny-fabric, and I was four years old. I think in some respects LT is pulling out some happy things from her own childhood and trying to incorporate them into this weird groundhog day existence, like recreating the original wedding that marked her escape. I don't think from LT and DT's perspective that DT was kidnapping LT, I think that was her parents' perspective. I think from LT/DT perspective she was being rescued and that wedding day and starting their own family was the happiest time of their lives...they just didn't know how to move on from there, didn't have the faith that it could still be good if you let kids grow up properly and allow the kids access to a dangerous outside world. So I think it's been a slide down, punctuated by events that forced even more extreme measures to be taken to protect what they'd built and to stop the fantasy collapsing in on itself completely.
I have two thoughts, one is 'how to boil a frog' and the other is about being backed into a corner. If you raise the temperature of the water a frog is in very slowly it won't be able to tell how much hotter the water has got, and will stay until it boils. Sometimes people don't start off 100% evil, they just take steps on a ladder, and each step might not be that far from the last. But if you combine that with being backed into a corner, your options become fewer each step you take, and so there's a combination of each step making the next not seem so extreme as if you looked at it as an A-Z leap, but also that the options are reducing each time a step is taken, and you can't go backwards, so you keep going forwards or sideways, but always digging a deeper hole, and each step makes it harder to get out of that hole, just like being stuck in a corner with reduced options.
I'm so sorry for the length of my posts in this thread.