So IMO here is a scenario that I really think could've have happened:
Complete rigor mortis takes up to 12 hours after death to fully set in, and starts with smaller parts of body first, i.e. eyelids, jaw etc., and larger muscles last. (
https://en.wikipedia.org/wiki/Rigor_mortis) With the time of death range given in this case, it is likely that full rigor mortis had not set in at the time that AS discovered RZ's body. When placing RZ on the ground, or any attempt to move her body, it is likely that one would place arms under the leg, as this is the natural way to carry/pickup/move a body. Furthermore, since RZ's hands were tied behind her back you wouldn't place her directly on her back. This could explain how the body was positioned with legs bent, and slightly on one side. It would make sense as full rigor mortis set in that the legs then became rigid in this position.
As for lividity settling in the back, if a body is moved within a certain time after a hanging death and placed in a different position, then the hypostasis in the limbs will fade and new areas of lividity can appear along the back.
Suicide is sad, tragic, and traumatizing for those left behind. It might not always make sense, and it is understandable that grieving family members do not want to think that their loved one chose to die. However, that does not make it murder.