Okay, since Holdon asked me nicely, I'll humor him here. He asked me to profile an intruder suspect, so here goes:
Using what I know, for the sake of argument: I'm afraid I still can't go along with your notion that it really was a foreign faction, nor do I think it was some random stranger. Hope that's all right. My intruder suspect is a man, full grown, anywhere from 25-50 years old (not really important) who knew JBR and her family. For talking sake, we will call him "Norman." (As in Norman Bates.)
He knew them well enough that he didn't set off any alarm bells. But he secretly had an unhealthy interest in JB. He killed her because he was afraid, afraid that if she went on the trip after Christmas she would get brave enough to tell her family about his obsession and what he was doing to her and he'd never see her again. In his mind, her parents didn't really "love" her, he did! He wasn't going to let them ruin their "love." So if he couldn't have her, nobody would. He would shut her up once and for all, and make sure her parents would suffer for not "loving" her. He wanted to save her from them. That's how I figure he justified it in his mind. It also might explain why the note seemed so angry at JR. "Serves you right for not appreciating her! You think all that money makes you better than me?"
Okay, that covers motive, from that perspective. Now here's the nitty-gritty. I think he knew their schedule. He used a key to get in. Maybe he was given one, maybe he stole one, maybe he copied it. Regardless, he was there while they were gone. He knew the place well enough to get around smoothly. He wrote the note. He had been practicing PR's writing, just to make it really bitter. Then he waited in one spot so her wouldn't go banging around the house. Maybe he was under her bed. Once they were all asleep (or when he couldn't hear anyone else moving around), he came out. He woke JB up gently. He said he'd like to "play" for a while. She said okay, but she was hungry. He decided to take her to the kitchen and let her pick what she wanted. She saw the pineapple dish and had to have it. Lifting her up by her waist, he held her high enough so she could pick some out with her hands, not actually touching the bowl. He took her back to her bedroom. Since the pineapple had digested, which would take a while (1-1/2-to-2 hours) he had to do something that would account for that time. So they decided to play their "special game." Without going into too much detail, he molested her for the next hour or so. But this time he was too rough. He made her bleed. He took the little underwear off of her and put the big ones on. He apologized for being so rough on her. He kept the little pair. After they finished their game, she had to go potty. She did, while he prepared himself for the endgame. He put her back into bed, saying he'd make sure she got to sleep all right. She turned over onto her stomach. WHAM! She never knew what hit her. He had used a heavy-duty flashlight he had with him. He could have left right there. But he figured to make an artwork out of her. He tied her wrists. But that wasn't good enough, so he strangled her as well. He made sure to pull it deep so it wouldn't come off too easily. He took a piece of tape off the back of one of her dolls and put it on her mouth. A work of art, "Norman" thought. Her parents will always remember her this way, and I can always imagine what they will look like when they do. Since it wouldn't do to just leave her there, he decides to really torture them. Make her hard to find. Let them think she's still alive for a while. It'll make it all the better when they realize she was under their noses the whole time. He took her down to the basement and left her there. He dug a blanket out of the dryer to wrap her up like a Christmas gift. The perfect Christmas gift for her parents, he thought. And he slipped away into the darkness. Whatever became of "Norman" after that is lost to history.
PHEW!
Okay, Holdon. I told you there would be two conditions. Coming here to do it was one of them. Now it's time for number two. Now you have to humor me. Now you have to come up with an RDI scenario. Doesn't matter who it is. It has to be the one you can believe. I hope you're game, because I'd hate to think I wasted all of this effort.