Donatra was turning the military against Shinzon, not on principal, but because he essentially said to her "Romulans are second-class citizens." Donatra wasn't on Shinzon's side from the beginning. It wasn't Donatra standing up there saying "Why haven't you attacked earth?" That was Suran. The plans against the Federation were not known. Suran thought it was to conquer Earth. Whatever his motivation to change, Suran is the one who changed his mind, not Donatra.