He was listed as "Accepted" before every other Academy applicant, and this was the same time his sister's status was changed to "Inactive." Given that Dhara is probably Force-sensitive, she was likely taken for training as an Inquisitor, with her brother's life used to secure her cooperation. Merei outright says they want him there, after all, and timing is too neat for a coincidence.
- The ending of Imperial Justice outright confirms that Leonis is sent to the Officers Academy on Arkanis as part of the investigation into the "Dev Morgan" incident and the Inquisitor's suspicions regarding him. However, when Leonis learns as much, he shrugs it off because it doesn't really end up changing things. He's made it to Arkanis and he still has to find a way to rescue his sister. If he can't, all is lost.
- An adaptation of The Secret Academy is unlikely, since Star Wars typically doesn't adapt other works into other stories (an exception is the Zygerrian slave arc in The Clone Wars, which was a comic series at first), and the show (as of this writing) is already at Season 3, at least a year after the events of The Secret Academy. Rebels material also usually doesn't go back to earlier times in its story like returning to Season 1 if we're now at Season 3. Zare and co. could show up as part of the Lothal/Garel Resistance though.
- Word of God is that yes, she was channeling the dark-side at the time.
- Confirmed, more or less. Dawn of Rebellion, a sourcebook the Fantasy Flight Games Star Wars roleplaying game states that "The increased crackdown on suspected resistance activity birthed a more formal resistance on Garel. Members of the Leonis, Spanjaf, and Statura families have helped to organize and lead the new Garel City cell and continue the resistance."