Spoilers
Oh my gosh! Your a master detective. I only read the novel up the the reveal of the the betrayal of the 1st prince! But yes you are very much correct! The 1st prince is a black mage that initially caused the fall of the kingdom in the Mc's 1st life. If i remember correctly, the 1st prince is one of those quite people that smiles at everyone but hates how humans are greedy and corrupted. I forgot why the 1st prince left the 3rd prince alive but in the new timeline when the 1st prince tires to destroy the kingdom again, he left the 3rd prince alive because he believes the the 3rd prince was the only exception, he didn't change growing up and wasnt corrupted by greed. And i remember reading another spoiler that the empire was manipulated to attack mc's kingdom and the princesses acutally did love mc and did not betray him on purpose( she supose the end up with him in this new timeline too)

Guys... is the ML's older brother, Cedric, dealing with Black Magic?
'Cause when Yuri was 13yo there's a scene where he asked his brother if his vision was getting bad and Cedric replied "not enough for you to worry about it".
Then, during the campaign against the Orcs, Yuri finds out that those dealing with Black Magic have problem with their vision.
Am I... reading too much into it?
When Yuri said he wanted to see Cedric's Briol, it cut to a scene with only the lower half of Cedric's face showing, and he gave a weird smile with the foggy glasses.
Do you guys think I am right? Did Briol Kingdom fall in the first life because of the first prince?