The manhwa did a shit job at adapting the story from novel specially in season 2. I suggest reading the novel if you want to understand it better
Heinry's people were responsible for the decline of mages in the eastern empire. It doesn't sound like he set the plan in motion to cause mages to lose their magic the plan was too complicated to be a quick short term thing, but he definitely had some involvement but it's sounds like he's stopping the plan because he has his empress. Trasha about to be exposed for passing her baby off as the emperor's and I'll bet money that hawk was a Western empire bird person spy. And Shovshit sent our Navier a expensive gift he promised to give her after they had their child, she's gonna send it back through Heinry.
I'm not even gonna act like I understand what the hell is going on anymore.... I mean, I love the characters but the plot is all over the place...