Just speculation.
1. Daniel was not happy at all with the games going on. If the PI is telling the truth he did not like the people that Anna was hanging with. Hmmmm..who could that be??
2. He was turning 21 in January. He could get his own lawyer ..make his own will ..and Stern would be out as executor.
3. Stern is a control freak. Eventually they want to be the ONLY person who has any influence in their marks life.
4. At this point Stern probably had talked Anna into saying he was then Hannah now Dannielynn's father. Instant next of kin if something happened to Anna and Daniel.
5. When opportunity knocks you take it. Daniel was just visiting. What would have happened if he made it back to the states. What would that PI find?
I think the question is if we truly believe Stern killed Anna in February why was he so frantic in December after her drowning mishap (suicide?). That would have been perfect for him..but why at that time did he say.."If you die I will go to jail." What happened to make her death in February expediate for him?
Ooops I misread your question. I believe that Anna could handle all of the meds. She probably needed them to function. When Daniel got ahold of them his body couldn't take it. The drink was so she could get her fix.