idk about the necklace being involved that does seem unnecessarily complicated but the last bit, about joffrey putting his cup down in front of the old tyrell woman is almost surely the point at which it was poisoned. Unless of course the entire point of having joffrey's cup handled by tyrion, sansa and gramma tyrell, before the poisining, is to confuse us into not knowing which one of them could have done it.
what i dont get is what would be the tyrell's motive for killing joffrey? They just lost a chance at having margaret marry the king, of their next born son be king of westeros....why would they want joffrey dead BEFORE the wedding? Seems massively counter productive to the whole obsession with forwarding house names.
Even if they were planning to betray him all along the tyrells had every reason to let joffrey live at least until the next day