Responses
the first is obvious, it's from the syndicate after all.
Then the store clerk, he's kinda delusional so I believe all he has are assumptions and is trying to get close to Reika to get to her father and prove it. It's a fan indeed.
The third is kind of a stretch. He's poor and works so he can't stalk. It's the more mysterious of the three, although not quite smart enough.

this is what i think happened and why so many ppl know about the murder
-blonde guy was sent from the syndicate
-convenience store guy is a fan of mc maybe he just assumed the novel mc wrote happened in real life or maybe hes a stalker and saw it happen
-fortune telling guy maybe he has real powers?
-boxing guy i think he stalked reika and thats why he knows so much.. maybe he planted cameras in her house or something