I would argue that it was Justinia who rebelled. The templars vowed to continue to do what they have always done. If the chief of police ordered everyone to set all criminals free, and make no more arrests, S/HE would be the corrupt one.
Lambert was Lord Seeker, and as such had authority over both seekers and the templars, he had the authority to do what he did.
Justinia is the head of the Chantry of Andraste as the Divine; it's a bit reaching for you to say she 'rebelled' when she's legally within her rights to lead the organization on a path she sees fit. Lambert, on the other hand, seemed incapable of doing his duty, since he continually acted as though he was a templar, rather than a Seeker.
I'm also pointing out Lambert's forces don't have legal authority because they defected from the Chantry like the mages did, and dissolved the Nevarran Accord in the process; both groups separated, so I don't see how anyone can argue that Lambert has the legal right. Asunder also provides insight into the character's motivation: "Lambert slammed the door shut and allowed himself a smile. He imagined the Divine reading that. Without the templars, the Chantry was toothless - nothing more than a bunch of old women armed only with words. What would she do? Try to convince the people, after ages of teaching them mages were to be feared and contained, that now everything was different?
"In three days the templar host would march on Andoral's Reach. With any luck, by the time he returned victorious the Chantry would have come to its senses and chosen a new Divine... one that would be eager to reach a new Accord with the seekers, placing the power much more firmly where it belonged."