The soiled panties that were seen in a crime scene photo on the floor of JBR's room (or bathroom) were inside a pair of pants, which were also described as soiled. We haven't seen THAT photo, but there is reference to it in PR's interview where LE is showing her that photo and asking her about it. I do not recall if the pants were the black velvet pants she wore to the White's, but if they were, and if PR discovered them solied upon return from Christmas dinner, that would surely be a trigger for the events that followed.
As far as the post-mortem urine release, it is possible that it occured after she was wrapped in the blanket and left in the basement. Though this release occurs right after death at primary flaccidity, JBR may have been THOUGHT dead but not YET dead when she was left there. The stagers may not have known it happened. However I have not seen any mention of the white blanket having urine stains, so this has to be considered when exploring this theory. The autopsy report states the long johns were stained on the ANTERIOR (front) of the legs and crotch, so this means she was lying on her stomach when she voided. However, she was allegedly found on her BACK in the wineceller. If we had only JR as witness to this, it could be suspect, but FW was there too and saw the body in situ in the room. I think if she had been found otherwise, he would have said it, because he did tell LE he had looked in the room earlier and didn't see the body. Now, if JBR had been left dead on her stomach, livor mortis patterns would indicate that, as well as indicating if she'd been moved after death. The livor patterns were non-blanching (fixed) and so from that we can infer she either died on her back or was placed on her back within 30 minutes of death (when livor begins).
Urine STAINED (as the autopsy states) is not the same as urine SOAKED but we have to remember that the autopsy took place the morning of December 27th, well over 24 hours after death and certainly long enough for the garments to dry. When he first arrived to pronounce her dead, the coroner made no comment about the clothes being wet or dry.
As far as the release of the contents of the bowel, this occurs after death only when the fecal matter is ready to be excreted, and then is emptied when the sphincter muscle relaxes at death. In JBR's case, this did not happen, as the soft green fecal matter noted in the autopsy was found in her intestines. This represented whatever she ate at the White's and the previous meals Christmas Day. The pineapple, being the last thing she ate before death, was still in the small intestine and still partially identifiable.