I agree. I think if it were Susan's blood they would have arrested Josh already.
But it may have been some other kind of bodily fluids.
I think the most plausible scenario is that Susan was angry and voiced her plans to separate and wanted Josh to leave the house. He probably
escalated things and needed to move her from the scene. I think he involved dad in that process somehow.
ETA: I just read a few posts above with a better scenario. Josh made her special pancakes, drugged her, and had planned it with dad.
I think Susan invited Jovonna over, so that she and Josh would be civil to each other. Can't argue in front of a guest.