You're right, UKGuy, that the wool shirt wouldn't be washable, unless some new type of treated wool which I've never heard of. I didn't remember that it was wool.
So, that leaves us with a perp-if-there-was-one stealthy trip up the straight ( not the circular) stairway to the parents' closet, separate and across the stairway opening from a door into their very large bedroom. See Appendix A, PMPT. Probably the door to the hall and stairway was closed.
If someone had gone up the circular stairway,it opens right into the parents' bedroom, and they'd have had to pass through there. He really did seem to know the house, if there was a he. Maybe the duffle bag tells us there was one. Is there any way to ask ST what was in it? He didn't include it in his book, not that I recall. But he may know, and might talk.
Looking at the floor plan in Schiller's book, seems like it would be very easy to get up there by means of the straight stairs w/out making any noticeable noise. Someone would have had to just turn right into an area that could be considered a hallwayand go through a bathroom, to their closet. There's another bathroom and two large dressing rooms, and at the other end of the 3rd floor,, directly off their bedroom, it looks like there's another, smaller closet, but it isn't marked closet. What else could that be?
The black shirt being WOOL gives us another possible motive for a perp if there was one, who may have noticed JR wearing it, maybe at the party on the 23rd or at the W's or Pasta Jay's on the 25th, may have been planning this crime and thinking it'd be very easy to get fibers from that shirt and PR's jacket. (Sounds like most likely it was the 25th?)