I disagree with your logic that, simply because they were not found in her drawer, that it automatically eliminates PR -- or anyone, for that matter (i.e. BR or JR). The only thing that the absence of the underwear from her drawer indicates is the following: that if they were in the drawer, they were (all) removed; and if they weren't in her drawer, they were somewhere else in the house. In both of these scenarios, someone from within the house had to know where the underwear was; and someone from the house felt the obligation to "dispose" of the remaining pairs. In addition, in either scenario, any of the three people who lived in that house could have grabbed the underwear -- it doesn't necessarily eliminate anyone. That's how I see it anyway.
We know that no other gifts were located in the wine cellar, so PR's account, for all intents and purposes and with regard to the gifts in the wine cellar, have been substantiated.
You bring up the idea that the underwear was used because it was the "Wednesday" pair and I agree with this idea, but I disagree that it would be BR who would use the pair for this purpose. If this is the reason why this specific pair was used, it was in order to convey the sense that the crime had occurred the night before (in other words, for staging purposes). The parents were the ones that were most involved in the staging element of the crime, even if you do believe BR committed this crime (which I don't, but I digress). The thought process of a nine year old wouldn't be to stage when the murder occurred, just as it wouldn't be the responsibility of a nine year old to specify a particular date on a loved one's tombstone, as the R's did for JBR.