As you say, they've built up Olenna Redwyne as a major character so I doubt they'll want to take away her shining moment, plus I think they'll want to leave it as a mystery for a while, for the watcher-non-readers anyway. They might try to tease him, Varys, Baelish, and her as options, they may even leave it open to doubt if Tyrion did it. But I doubt they'll change it. Although I don't think it has ever been confirmed it was Olenna beyond a doubt and Baelish is obviously an unreliable narrator of events. So I won't be surprised if they hint at it as an possibility, and I wouldn't call it impossible by any means, but I don't think it is in the cards.

Well they've proven they're willing to make major changes, and it really doesn't make much difference who kills Joffrey, it is arguably Stannis and Melisandre who are responsible anyway, all that really matters is that he dies and Tyrion is accused of it. Hell Tyrion could actually be guilty of it and it wouldn't really matter. It can't be Cersei and she needs to believe it was Tyrion, it doesn't really matter after that.