@Talh
1. just because succession war benefitted demons doesn't mean it's wise to be automatically assume it's them: humans rulers get killed all the time because of humans' political maneuvers/citizens' dissatisfaction at the ruler. Kail found no evidence of king remonas collaborating with demons in both timelines, and it's perfectly believable that a daughter-killing douche like him would think he would benefit a lot from weakening his rival nation : the galgan empire.
2.
Incorrect,
only one occasion in human territory: in the 2nd arc against the 2 demons. Also, this was a covert operation to get a heirloom ordered by peaceful Louisa, not warmonger Hornless who would rule later.
Also as I've mentioned, Kail was busy trying to increase his fame/influence as a hero and strengthen humanity as a whole: he really doesn't have the time to investigate the details of eldrand's previous assassination--an investigation which may turn out to be fruitless since detective work isn't really Kail's forte, so even if Kail did suspect possible demon involvement there would have been no point in overthinking things he doesn't have any control over.
Lastly, Kail has changed the future a lot and really derailed Hornless' plans by preventing Hornless from taking out Louisana and ruling over all demons (yet), so it's getting harder and harder for Kail to predict what will happen/what moves Hornless will make: it would be
ideal if Kail can sabotage Hornless' plans but there is no way Kail is relying on that kind of flimsy hope especially when he really doesn't know much about hornless.