UKGuy, I'd be interested to hear your theory about the Wednesday underwear.
But I think any speculation that the Wednesday pair was chosen because someone at the Whites had seen them, perhaps by assisting JBR in toileting etc., is kind of iffy.
True, PR and JR said JBR was 'zonked' when they got home and that they didn't wake her to put her in pajamas. But if they had absolutely HAD to account for a different pair of underwear, couldn't PR have claimed that she replaced the underwear when JBR's pants were removed and the long johns put on?
Unless I'm mistaken, PR always made it sound like she'd removed the velvet pants and THEN put the long johns on, not like she took off the velvet pants and left the long johns, which were underneath, there. But if she took off the velvet pants and then put on the long johns, it would be easy for her to say, if the Whites or anyone else mentioned different underpants to the police, that she had replaced them at bedtime. She has the perfect reason for doing so; her little girl was known to have 'accidents'. So why would it matter if the Whites or anyone else had seen the Wednesday pair?
IMO, the only person who needed to be fooled by the Wednesday pair was PR, in the first staging JR might have arranged. And I think JR would have known what gifts PR had been working on, because I think she came home Christmas night complaining that she still had to get gifts wrapped and ready for the Charlevoix trip, and that she was the sort of person who would have listed them all off out loud just to show how much more work she had to do before she could go to bed that night...which is why I think that JR, and not PR, was in charge of bedtime for both JBR and BR on Christmas night.