goldengrasp, I’m not sure I’m the right person to ask about the Tyrells’ wedding plans, but I think the goal was to keep the Tyrells as far out of it as possible, to avoid any blame. They didn’t want to risk the possibility of one of the cousins being implicated, if someone realized that the hairnet was the source of the poison and then remembered who was wearing it.
Dontos gave Sansa the hairnet at the end of ACOK, while Margaery and her grandmother didn’t even arrive in the city until the beginning of ASOS, which is something I still don’t really understand. Why was Littlefinger planting a poison hairnet on Sansa before the Tyrells were seriously planning to kill the king?
“It’s very lovely,” Sansa said, thinking, It is a ship I need, not a net for my hair. “Lovelier than you know, sweet child. It’s magic, you see. It’s justice you hold. It’s vengeance for your father.” Dontos leaned close and kissed her again. “It's home.”
I don’t think the Tyrells were planning on using Sansa in their plans as of ACOK, because they wanted to wed her to Willas and claim the North through her, and you can’t really claim the North with a bride wanted for regicide. And I don’t think the Tyrells were really ready to kill Joffrey until ASOS, when Olenna and Margaery sat down and had their little chat with Sansa and she admitted that Joffrey was a monster.
So maybe Littlefinger was just doing his usual in ACOK and planting the seeds of chaos, with a plan to entrap Sansa and make her reliant on him one way or another for when he eventually whisked her out of King’s Landing. And then in ASOS, when Olenna needed a poison, and Sansa had been married to Tyrion and was no longer of use to the Tyrells, maybe Olenna went to Littlefinger and they took a walk together and he said very carefully, “Have you taken a closer look at that hairnet on the Stark girl? Do you realize what those stones are?” And then the Tyrells unfriended Sansa, because they couldn’t be the friends of a future “criminal” and draw any suspicion on themselves.
And I’ve seen theories that Olenna also wanted to steal Sansa away in the resulting chaos with hopes that Tyrion would be executed, but I’m not sure the Tyrells’ plan involved directing the blame at anyone in particular, so much as just directing it away from themselves, because, yeah, as you mention, Oberyn could have been implicated:
“I have much to thank your sister for. If not for her accusation at the feast, it might well be you judging me instead of me judging you.” The prince’s eyes were dark with amusement. “Who knows more of poison than the Red Viper of Dorne, after all? Who has better reason to want to keep the Tyrells far from the crown?”
It was Littlefinger who arranged for the jousting dwarf mummers to rile Tyrion up at the feast and direct the attention to him.
So why include Sansa? I think it was just convenience, that she unwittingly had the perfect delivery system for the poison. As to why Olenna made this arrangement with Littlefinger, or how much of an arrangement it was, well I think that still remains to be seen. Littlefinger was surely advancing his own creepy, predatory agenda, whatever it was, but maybe someone else who has thought about this more could explain better.