That's the agenda I talked about. I'm not sure of the semantics or details, but I just read on the wiki that the Martells have ceased all sorts of trade and relations with other houses since Targaryen rule fell. The arranged marriage is the only such thing and we both know there are so much more to it. I think Doran is starting to play the game. Myrcella is not just a Lannister, her last name is Baratheon. If Tommen dies, she (and her husband) have the strongest claim to the throne.
There are a number of issues here.
Doran was always playing the "game". The only people that never really were was Robert Baratheon and Eddard Stark.
Myrcella is just a Lannister. The noble families all know the rumors of Jaime/Cersei's relationship, and I'm pretty sure the "smarter" members know that there was NEVER a fair haired child in House Baratheon history let alone three in a row. If Jon Arryn and Eddard (I'll never understand the "Ned" thing) Stark were able to find actual clues that spelled it all out, then it is also possible that others have also come to the same conclusion without the whole getting murdered part (honor tends to do that).
Robert named Joffery, Tommen, and Myrcella as Baratheon's, but that was only made with the assumption that they were actually his children. If evidence came out that proved that they were actually the products of incest Tommen and Myrcella would be named bastards (if not just Lannister's) before they were executed along with their parents for the crime of incest by the High Sparrow.
It should also be noted that Littlefinger handed the information of the parentage of Cersei's children to Eddard Stark, and he handed some information over to the High Sparrow which helped lead to Cersei's arrest.
A bunch of idiots in masks with daggers can almost do it though.
Which is why all Stannis would need to take on the dragons would be a couple dozen balista!