Maybe someone with a better memory can clarify, but I remember Mary taking a pill in front of someone (Quinn maybe). If toxicology is done and there is mention of Mary being off her meds, Quinn (or whoever was in that scene) can say that she saw her taking pills. If they are going to go the route of blaming it on Shay with evidence, I see this the way it going. To me, it's all on Shay. Mary would have had different reaction to the day if he meds hadn't been tampered with and no matter how much Shay tries to throw the blame elsewhere, none of what happened would have occurred if Shay hadn't switched her pills.
I was very lucky in that I never saw 'wait for the last 30 seconds' promo so the ending did shock me. At first, I thought they were going to go a different route with the ex by saying he never did hit her and that it was all in Mary's head, but then they had him just...wow he took some really low shots.