I don't think that's enough to make him the "most dangerous man in Dorne," though. It could mean he's aware of what really happened (R+L=J) but I think GRRM will want to keep that a big surprise that only Howland Reed and Ned Stark knew about. We know it's not because of his fighting prowess; we're told he's good with a sword, but he fails to kill a little girl and then quickly flees. Not exactly Jaime Lannister-level. Oberyn also referred to him as "poison," and there's the fact that he tried to kill Myrcella in the first place and is likely the one who told Doran about the Queenmaker plot. That suggests to me that he and Doran already had some kind of relationship, otherwise Darkstar wouldn't bother going to him to sell out Arianne.
No, he's dangerous for something else. The real heir to the Iron Throne, who also happens to be hotheaded, arrogant and what looks to be another Viserys situation sounds pretty dangerous to me. As to why Doran doesn't marry Arianne to Gerold Dayne instead of (presumably) fAegon, I don't know. It's not as if House Dayne is too low for her, and thanks to Arthur Dayne it's relatively well known throughout Westeros. My theory is that Doran is supporting fAegon over the real Aegon because he wants to check out whether this supposed Targaryen taking Storm's End has a large army, is likable and would be a good king; from the looks of it so far, Darkstar would be a terrible king and Doran knows this.
