Or just have Margaery marry Tommen who is next in line because that would make the Tyrells look even better to the people while giving Margaery a lot of power in a much safer setting since Tommen doesn't seem to be a madman...at least yet...
But I keep going back to
Littlefinger being involved though due to Sansa being helped to flee the city. I posted about it a few times explaining his motives and his goals, and it just makes sense to me. Repaying Joffrey for Catelyn's death among other things, including Ros's death, but it also gives him Sansa who he wanted in the past since she can be used in the North while also being the daughter of his beloved Catelyn.
Maybe he'll keep her hidden a while before making his own moves.
So many people have something to gain with this move and this just seems like something Littlefinger would do.
Oberyn definitely wouldn't do something like this since he seems very vocal about his hatred for the Lannisters and wants to kill them himself and poison is seen as being very cowardly and lacking any honor. Who else would use the drunk guy as a patsy to cover his/her tracks and want Sansa to escape as well?
I also try to look at it from a story-telling perspective since Littlefinger has gone missing since going to The Vale. He was a major character in the past and I don't see him going away anytime soon, so what better way to bring him back than this? Maybe Olenna and LF teamed up for this joyous occasion!