YOSSARIAN said:
Doesn't seem like it should be that easy, but the CIA has been employing this technique since day 1. All it takes is a charismatic agent (Taena), a sufficiently credulous 'asset' (Cersei), and a believable fall-guy (Senelle). Actual proof is optional. I read this in a book that contained declassified CIA reports, one of which was a list of sucessful contact and interrogation techniques.
Wow, that's scary!
Intelligence gatherers just seem to be a bunch of hyper-paranoid rumour-mongers, huh?
Anyhoo, back to the point. I don't think Taena was a Tyrell spy - think about it, if she was in Cersei's circle of trust why didn't she try harder to divert her from them?
Instead she actually gave her information which led to Margery's arrest. At the very least, she knew about what was going to happen in plenty of time to warn her what was coming and give them time to hide her.
I don't think the Stannis connection fits either - even if she was in cahoots (i love that word) with Saan, he's a mercenary who'll switch sides at the drop of several coins.
If I was to suggest a guiding hand, it would be Illyrio. He has motive (Targaryen loyalist or so it seems...), the connections (Varys) and is the only one to benefit by a power vaccum at Kings Landing and furthering the chaos in Westeros.
The Tyrells had more than they could have dreamed off, they would
not have rocked the boat. The Dornish were up to their own internal schemes, the Northmen are scattered & leaderless, and the Iron Islanders just want to pillage the crap out of everyone.