And that was a big mistake.
I don't see the need for the garrote to balance the absence of visible head injury. Especially if she already appeared dead, or they assumed she'd die imminently. They knew the head injury would be discovered sooner or later. There is no need to "explain" the death is some visible obvious way.
Right. So the panties didn't really hide anything. If the objective was for the re-dresser to hide injuries prior to the "discovery" of the body, the long johns served the purpose. If someone had pulled down the long johns the panties -being too big- would probably have come down with the long johns. Eve if the panties somehow stayed up they'd be obviously too big prompting the question -WTF? So the panties didn't really provide a secondary level of hiding. Possibly they were meant to be discovered later and lead investigators astray? Or, they may have been used as part of "undoing" as Cynic suggests, but they serve no purpose in hiding the injuries, being under the long johns.
The panties were needed to hide the evidence of a sexual assault (blood on the original panties), not the assault itself. The original panties were removed/hidden/destroyed. JB had panties on that day, over which her long johns were put when she got home. As Patsy said, NO panties would be unusual, and they wanted her to be the way she was when they readied her for bed.
It wasn't the injuries they needed to hide. They were internal for the most part. They needed to hide the fact that the panties she started the day with were no longer on her. They needed to replace the panties she already had on.