Specifics aside, I will eat my hat of Little Finger wasn't the grand architect of Joffrey's assassination. His absence from the show thus far hasn't gone unnoticed, which I wouldn't be shocked to learn was a hint of some sort. The showrunners are clearly trying to plant various culprits by reminding us throughout the episode that pretty much every character present at the wedding had some motivation to do Joffrey harm. Now I'm sure a few of them were indeed co-conspirators (namely the Jester - I forget his name, as well as Margaery and Olenna), but yeah, Little Finger has to be the puppet master here.
I agree with other theories that the necklace likely played an important role as well. I doubt Sansa had anything to do with it. If my theory is correct and Little Finger was behind it, there's no way he'd risk implicating her (and potential harm coming to her should the plan go wrong). Whoever was actually responsible, it definitely seems like Tyrion was meant to be framed for the murder.
I agree with other theories that the necklace likely played an important role as well. I doubt Sansa had anything to do with it. If my theory is correct and Little Finger was behind it, there's no way he'd risk implicating her (and potential harm coming to her should the plan go wrong). Whoever was actually responsible, it definitely seems like Tyrion was meant to be framed for the murder.