I think you do a great job of building a narrative and you address a lot of questions. I have lingering questions as to the staging. Why the SA with the paint brush handle ? In my mind that length of abuse to a corpse wasn't necessary to imply SA by an intruder. Why not just uncloth her or remove her bottoms? I think the staging could have been achieved without that element. If you are of the theory BDI by accident , the SA staging is more monstrous than the accident.
It may be accurate but something to me is off. They went from victim to monster IMO.
Thank you!

I try my best.
If you ask what I think of this, I believe that the SA was a separate occasion from the staging itself. There is no clear evidence that there was any SA that night. Yes, she had been abused as there were signs of chronic SA and acute SA that had happened at some time shortly before, but we do not know exactly when it happened. Yes, there was blood that had been wiped down from her thighs and that was seen with the UV light, but again, we do not know for certain that this blood had resulted from the SA in the time period between when they got home from the Whites and when she was killed.
She could have been bleeding at any time between her last bath and the time she was killed. And so she could have been wiped clean also in that time period. We know she did not take a bath that night after arriving home from the Whites, so she could have possibly been abused the night before, in the morning of 25th or at the Whites. The size 6 underwear has never been found, so it is again possible that she could have bled a lot more on them (and maybe that's why they are not found). The size 12's that she had on had only a few small droplets of blood, witch could indicate that no SA happened during the staging. It is a possibility of course that it did happen, but I somehow tend to believe that no SA happened during the staging part. And the reason I think that is exactly what you said - the staging did not need that part. And, they were still parents who had just lost their daughter. I just can not believe they'd be thinking of sexually assaulting her body for the purpose of staging.
We also do not have any factual forensic evidence to connect the paintbrush to SA. Yes, there was a very small birefringent cellulose material found, but that so far is the only thing that connects the paintbrush with the SA and we can not say for sure that the paintbrush was the object used to penetrate her. Again, why? I still see other possibilities how it could have gotten there as well. And, IMO, if she was SA'd with the paintbrush I'd think there would be a lot more evidence of it happening than just this one small found material.
So I really do not see SA as part of the monstrous act here. I think they (probably John, with Patsy being present) just strangled her down in the basement, where the urine stain was found, because they believed truly that she was already dead and that made it "easier" for them to do so. Ugly thought to say, I know. They were her parents and they loved her. I believe that they were in absolute shock and devastation, but as they came to the mutual decision that all this needs to be staged and they all need to be/stay the victims in this crime, they needed the "monster" who came to their house. IMO They had to do strangle her, write the note, tie her hands and place the duct tape because there had to be evidence of that act, of a "monster".
I did write more of how I see the night unfolding in the post explaining my theory.