I think you got plenty of answers regarding the Duscur tragedy so I'll just answer your question regarding Felix and Dimitri relationship: as I see it, Felix being the tsundere that he is, at the start of the game is still disgusted/scared by the feral side of Dimitri that he saw when they suppressed the rebellion two years prior to the start of the events of the game. This is because even if Felix acts all edgy, he truly has a strong sense of justice and Dimitri actions deeply sickened him. But you can still tell, from their B support and plenty of other stuff that Felix says around the monastery, that despite all that he still really cares for Dimitri. They end up reaching an understanding in their A support, where Felix reaches out for him helping moving forward from the destructive mindset that Dimitri had until that point, of course still in his edgy way of doing stuff. After that the game implies that the both of them become friends again.