.......good point, forgot about that. Technically, this makes Celence a traitor, too. Though I would argue that since Gaspard has not been crowned, and the duel lacks any witness that both sides would believe, there's no way that the duel remains valid. In which case, the throne is still Celene. By law, anyway.
All this "rightful" heir/claimant/monarch business bores me as much in Thedas as it does in Westeros and there are a bunch of excellent quotes about the futility of determining "right" and it's significance, like this one:
"Tell me, what right did my brother Robert ever have to the Iron Throne? Oh, there was talk of the blood ties between Baratheon and Targaryan, of weddings a hundred years past, of second sons and elder daughters. No one but the maesters care about any of it. Robert won the throne with his warhammer." -Renly Baratheon
You can say you're the rightful king or empress or whatever as much as you want but that doesn't make you 1.) the best for the job, or 2.) the one actually on the throne. As far as I'm concerned, if you take it, it's rightfully yours. The ruler makes the laws.
Of course they do. And right now, Celene is the lawful ruler. And nobles DO care about the laws. The laws are all they have to protect themselves from bigger, more powerful or more connected nobles. The only way lawful succession can be defied in an advanced society is if you have the power, either politically or militarily, to do so. Otherwise, the nobles and the commoners default back to what the law says. And if the law says a six-month-old infant is the rightful Emperor of Orlais, and no noble has the clout to make it otherwise, then that baby is the emperor. Of course, he/she will need a good protector or two to prevent those deadly 'illnesses' that strike poor children as they lay in their cribs.....
I'm not saying you're wrong, but I think you underestimate the importance of succession law in a fuedal society. Those laws may be fluid and easily changed, but nobles are hesitant to defy the law of the land without good reason or ample power behind their defiance. Gaspard happened to have enough people behind him, and the balls to try. Also, a surprising amount of righteousness. I think he truly feels that he is best for Orlais, and that Celene's continued rule would only lead to a ruined Orlais. I honestly didn't expect to like Gaspard and hate Celene as much as I did.