Before the retcon, Lux also used to be a spy/scout for the Demacian army, infiltrating into the enemy territory. Her Light Magic helps her escape from chasers, and I think she would use it only when she is forced to, otherwise risking to be observed by her own kind. I think she will still keep this role in her military life.
As about her Magic, I like to think no one, not even Garen, knows of her Powers. The idea of her being the "Witch" in the middle of the "Inquisition" makes her story a bit more interesting, and actually gives her more character than a "brain-washed-military-mage" that she used to be. Now she is fully responsible for her own actions, decisions, and not just "DEMACIA!!!".
And I hope she will actually be the one who exposes the "darker" parts of Demacian Society. I think she is the only who can present a somewhat neutral stance about the military, the duties of the high-class families, and the probably exaggerated rules.
About freedom, I think she will want freedom from the military a lot more than before. The Demacian military is formed mostly of Guardians, because it doesn't invade any nation. The fact that Galio awakens when Magic appears makes them very wary of it, and consider it a destructive element that must be stopped. So, if the military discover Lux's powers, she may be as well considered doomed.
This way, in the upcoming lore, while she may be forced to join the army , Lux now wants more than ever to just run away from everything dear to her, in order to be free to use her talents, and, most importantly, survive. We know her hiding cannot last forever, and if she doesn't choose to run away, her VO lines towards the enemy Demacians will come true : they didn't come to just have a fight, they came to hunt her down, like any "Witch" that threatens their Society.