Here's a brilliant idea; kill lady Mantillion instead of waiting for her to kill you.
Or here's a better idea; forfeit your claim to the throne and flee as far from Orlais as you can. No one forced Celene to play the game; she wanted the throne and she killed her own people to claim it.
As someone already mentioned; Celene had no support structure whatsoever, aside from Lady Mantillion. The lady wasn't actually her rival, she was more akin to a sponsor. However, as much as Lady Mantillon was her sponsor, Celene was the lady's tool, and she would have been disposed of at the first sign of weakness...or, at least, that's what Celene had been led to believe after learning the rules of the game. If she had killed her benefactor, her ONLY backing to the throne, Celene's life would have been forfeit for all she knew.
Forfeiting her claim to the throne would have been seen as nothing but an empty promise, a ruse to lull them into a false sense of security, in the eyes of the Game-playing nobles. And moving to Ferelden? Where would she go? She had nothing in Ferelden. Sure, she could have sold her family estate, but I highly doubt an Orlesian like herself would have been able to buy land in Ferelden so soon after the war. An Orlesian noble has little to nothing outside of Orlais. Celene claiming the throne was potentially a life or death situation for her at the time, it was do or die.