Morgan says that it's enough to turn just one Prior and it would effect all the Priors because they're all connected telepathically.
But in season 9 Teal'c convinced Garak (when he was a Prior) to turn against the Ori. it didn't effect any other Priors. Also Garak burst into flames, and it was said it was because the Priors' brains were programmed by the Ori to do that so that a Prior would die if he ever turns against the Ori. It didn't happen in the movie either.
Another plot hole is Tomin's story. Tomin already turned against the Ori in season 10, so it seems strange that in the movie they have to again convince him to turn against them.
he already betrayed his Prior to save Vala. It is highly unlikely the Prior would not know about this and would put Tomin as a commander in the army again.