The weight on Olenna isn't just because of the missing gem or whatever, but also (from my perspective) an observation of how this and the last episode were cut together in relation to this. Bad narrative has twists and other bullshit pulled out of nowhere with no substance or reasoning in retrospect. Good twists play the pieces right before your eyes and it only becomes obvious when you have the "gotcha" moment. Roose's betrayal of Rob is an example of this: his house has a rough history, Cat makes it clear he cannot be trusted, and his military policies butt heads with Rob's on several occasions. Again, Theon being flayed. The mystery is who is this guy and where the fuck is he. Meanwhile Theon is on a gigantic fucking flaying cross, while being flayed, and is continually taunted by a horn, the exact same horn blown by Bolton's men when they surround Winterfell.
So much emphasis was put on fucking necklaces and Olenna's disgust for Joffrey and Lannister bullshit in general was prevalent in the last episode and throughout the last season. Yeah sure, it could be anybody, but when freeze frame shows a fucking necklace gem disappear after Olenna is near by, the same Olenna who made a fuss about shitty nicklaces prior to the wedding, the same Olenna who knowingly loathes a sociopath marrying her granddaughter, etc etc. Come on. Good story telling, which Game of Thrones almost always has, suggests the killer can be implicated or at least narrowed down to a small pool based on events so far. Bad story telling suggests that the killer was fucking Hot Pie, cos pies. Or the kid sucking on his mum's tit, because...I dunno, tits.
I know a dude who I'm 100% positive hasn't read the books, and even he is hedges his bets on Olenna due to how the scene was cut. I don't think there's much plausibility to suggest anybody else specifically was involved. Maybe Olenna acted alone, being the craft mistress of awesome she is. Or maybe she had helpers, like the drunk, that will be revealed. That's the big picture that we don't know, but the writing on the wall for the time being says OLENNA.
Meanwhile, the book spoilers people are probably laughing at us because it was indeed Hot Pie.
I said this earlier, but Game of Thrones continually makes a point of how fucking hard it is for anybody of name and status to do anything without a spy picking up on it. This is like...one of the most prevalent core themes, integral to so many developments and twists. This exact episode makes a point of it, both Jamie and Tyrion reminding us how easily information slips out. Shit, even Lorris reminded us, hitting Jamie with the ol "yo you fuck your sister" shit. Nobody can shit, eat, breath, fuck, or do anything without someone noticing because movements of the rich and powerful are valuable commodities and essential political tools.
Yes, poison is small. But I think trying to smuggle in a poison that powerful, that active, and that deadly would be something watched out for by parties who know that information would be valuable. How does royalty get a hold of such things? How does a guest at Kings Landing acquire what seems to be one the most secretive and potent killing tools, without anybody noticing, and get it into a wedding.
Nobody suspects or watches a drunken nobody. A nobody in the strictest sense: nobody knows him. Even Sansa doesn't recognise him at first. The logistics are still murky, but it makes a lot more sense to use a tool that'll go unseen than to just piss away risk doing it yourself.