There are a few possibilities here. One, is that she may have wet the bed Christmas EVE, and Patsy didn't change the sheets Christmas morning. The sheets that were on JB's bed were taken into evidence, and creatinine was found (dried urine). By the time LE took them, the sheets were dry, not wet. Dried urine can't be "dated" as far as being able to determine when she wet them. This goes along with the theory that JB did not go to bed Christmas night.
Two- she MAY have gone to bed and wet the bed, but still had enough urine produced after that to be able to void her bladder at death. This would be especially true after eating the pineapple, and we don't know if she drank water or anything. But I tend to agree with your version, with her never going to bed that night, as I stated above.
My thoughts on why the white blanket was NOT urine stained is that she died in the basement, face DOWN on a carpeted area outside the wineceller. Then she was placed face UP on the blanket, which was pulled out of the dryer and placed in the wineceller for the purpose of placing her body on it.
And yes, the livor pattern indicated she was placed on her back and left there, within 15 minutes or so of her death. If you look at the autopsy photos of her bare back, you will see a thin white "stripes" effect on her back. This is how livor patterns look when there is pressure during the "blanching" stage. When livor is new, the body will "blanch" under pressure from anything. This is the blood moving aside under the skin when the skin is pressed or anything presses against it, even lightly. Live people "blanch", too- just press your finger into your leg or arm and you'll see it.
But when blanching is seen in a corpse whose livor is fixed (aka non-blanching), as noted in the autopsy report, those white stripes happened after death and came from the folds of her clothing or the blanket beneath her back as she lay there. In a live person, the blood will seep back into place when the pressure if removed. But after death, the blanched area will remain in place. After livor has progressed to the NON-blanching stage, nothing that presses in the flesh will make a white mark. So there is a short time frame for her to have been placed on her back and those marks to be made. If she'd been placed on that blanket after livor became fixed, there would not be those white marks on her.