I'm starting to think you're right. After thinking about the lack of blood in the house, the strangulation first, and the possibility that the girl came downstairs to get some pineapple out of the fridge, it hit me that she may have encountered an intruder who had been hiding in the basement the whole time and who had written the ransom note while they were at the Christmas party.
My brother suggested that Patsy may have been drunk that night and that is why she was wearing the same clothes the next day. As far as the fibers from a jacket "remarkably like" the one she was wearing found in the ligature, it may simply be that the rope was taken from the house that had prior contact with that jacket.
In any case, the forensic evidence seems to point more to an intruder than to anything done by someone in the home (or supposed to be in the home) since the asphyxiation happened first and there is no blood in the house.
The nature of the asphyxiation is such that it seems that a 9 year old would have a hard time fashioning a ligature like that, twisting it with that much stength and then bashing her head in with that much force (the only thing that would allow it is a baseball bat - but it too would have had traces of blood or bleach on it). No, this was an adult male; methinks now. We'll see.
I don't think Patsey wore the clothing all night. I THINK she changed but in the morning when she woke up in her pj's/or nothing, she had to put on some clothing quickly while the police were on their way their so she grabbed her clothing from the night before which was likely thrown on a chair in her room after she got home from Christmas night. No one would have had time to find a new outfit and put it on when they are in the same panic as the Ramsey's were. She just grabbed the clothes and put it on not thinking about a thing. It makes perfect sense.