Note: I have not read the books, so please don't comment with book spoilers.
Either Margaery or Olenna killed Joffery by slipping something in his drink; my money is on Olenna, with Margaery colluding as Olenna is less like to raise suspicion. Trying to poison the pie would had been too dangerous; too many variables since I assume everyone was meant to be eating it. Far easier to make a targeted strike on his drink.
The necklace last episode that the fool gave Sansa was from the Tyrells. I don't think the necklace Olenna wanted from the servent girls was destined for Margeary (we never saw it identified on screen), it was for Sansa. They arranged it so the fool would gain Sansa's trust (I think that was the one she was wearing at the wedding) and would (hopefully) run with him when he said so.
The Fool has been contracted by the Tyrells to get Sansa out to be taken back to Highgarden. The Tyrells have referred to Sansa as the “key to the North” and they want to use her to rally the families in the North to claim it back. They would marry her off to Ser Loras.
I'd be interested in non-book spoilery thoughts on this...
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