Okay, Fiora's updated lore is good. But it doesn't explain why she hates Lux and not Garen.
Can we get more context as to why Fiora only hates Lux and not Garen? If it were just because Lux was a Crownguard, she'd also hate Garen for the same reason.
My guess: Lux was the one who caught Fiora's father trying to cheat. Since Fiora had to personally kill her father afterwards to save her family from exile, she's pretty pissed off at Lux for the whole thing.