i don't like the purple wedding at all, plot wise. It feels very weak. Red Wedding made sense - Lannisters get what they want, they get Roose as a new ally, and the Frey's get the blame down in history for fucking King Robb in a despicable way.
Purple Wedding makes absolutely NO SENSE. Olenna poisoning Joffrey before Marge has a baby in her has the potential for Marge becoming worthless, it was complete plot-armor-luck that the Lannisters agree to marry Tommen to Marge, it seems more likely for them to say "fuck that shit" and marry him off to someone else, making Marge the Unlucky Queen for a day. Getting rid of Tyrion and Sansa is pretty worthless to the Martell's, they should be concerned with Tywin's power and Jaime becoming the Lannister heir, not fucking around with Tyrion. Olenna keeps implying she is super smart and cunning but that whole sequence was boneheaded and depended on too many people outside her full control, especially if she worked with Littlefinger.
The only saving grace really is that Littlefinger is a goddamn erudite liar, so its entirely possible the stuff he has said in the books to Sansa are total lies and that we still don't really know the truth of what happened or who did it exactly, and that Littlefinger was not really involved at all (except to know it was going to go down) and took advantage of the situation to get Sansa out. i.e., Olenna was not working with Littlefinger but Varys instead to poison Cersei (or Tywin) but Littlefinger found out and manipulated events to kill Joffrey and bring out Sansa, sowing chaos.
Unfortunately I think the series is gonna explain next week that Olenna was working with Littlefinger and Joffrey was their target.