I might be wrong about this but I'm gonna share anyway, lol.
What if the point where the timeline of seasons 1 and 2 changes significantly from what Lu Guang experienced before is when Cheng Xiaoshi dives into that photo of Liu Min in the parking lot and hears Emma getting killed and he screams, which stops Liu Min from killing her and later causes Liu Min and Emma to fight while he drives and thus the accident happens?
Which, I guess, would also explain why Lu Guang was so mad that Cheng Xiaoshi dove into that picture alone and why he was surprised when Captain Xiao Li told him that Liu Min couldn't be the murderer as he has in a wheelchair because of a car accident he had on the day Emma died. Because until then, Lu Guang took Emma's death as if it was always meant to happen. But it wasn't meant to happen with Li Tianchen's (and probably Cheng Xiaoshi and Lu Guang's) intervention. Which would explain why he was also surprised by the appearence of "Red Eyes" in the dive from the season 1 finale. And by his own stabbing, which he took to mean that the death node transfered from Cheng Xiaoshi to himself.
Perhaps the change was that Liu Min, after the crash, called to Li Tianchen for his assistance and that's why he possessed Emma and killed her. Which is what led to Cheng Xiaoshi and Lu Guang getting directly involved with "Red Eyes".
And it could also explain why Lu Guang seems so out of the loop in season 2. I mean, okay, he somehow knew to take that picture that Cheng Xiaoshi would use to possess Lu Guang and save his past self from Li Tianchen, but what if he took that picture, not because he did so in a past timeline, but as a precaution. To check what was going to happen in the next 12hrs since Cheng Xiaoshi, Qiao Ling and Captian Xiao Li left his hospital room. Though, I don't think he was able to see the 12hrs then, considering that just before, he couldn't see many hours into Li Tianxi and her mom's photo. So, he probably only saw it until the part where Cheng Xiaoshi is being taken to the boat by Li Tianchen and, probably, when he gets possessed by Cheng Xiaoshi. Which also would explain that he, most probably, wasn't expecting for Cheng Xiaoshi to get shot in the tunels by Qian Jin (though, tbf, even if Lu Guang expected it, he still would've lost it at seeing Cheng Xiaoshi getting shot, I think).
So, my guess is that Cheng Xiaoshi's dive to the day Emma died led to the timeline diverging from what Lu Guang knew from the previous ones.
Though also, I know that in season 1 it's stated multiple times that other changes also led to Liu Min's car accident and that Cheng Xiaoshi believes are the changes that led to Emma's death, specifically the changes Cheng Xiaoshi did by texting Emma's parents and getting Xu Shanshan on the news because she beat up a creep (though this last one doesn't affect Emma's death, obviously, as it happens after she is already dead, but it's one of the significant e.g. that are given to lead to the conclusion that the whole ordeal with "Red Eyes" and the boys getting involved in it was caused by Cheng Xiaoshi's interventions). But those could just be "small changes" and Emma's death node was the important thing that was mainly changed because of Liu Min getting spooked, not finishing her off and thus causing the car accident when they fought. Though, the node still found a way to happen, aka by getting Li Tianchen involved. Perhaps in the original or previous timelines where Lu Guang and Cheng Xiaoshi get involved in Emma's case, Liu Min was supposed to kill Emma without any interruption and wouldn't have needed Li Tianchen's help to commit the other murders after Emma. And thus, if the boys helped in the investigation in previous timelines, they just had to confront Liu Min and help the police catch him. And Liu Min wouldn't have had the "I was in the hospital the time of day Emma died and I can't use my legs" alibi.