The bell was probably used to cover up the sound of an attack being launched from the tower. Since POV had a direct line of sight, state of the body, plus mowed grass, and no other physical traces, my guess would be a windblade-like spell.
The dad was kind of dense and dumb before in the earlier chapters but this arc has just shown that he could be dumber. Like I get that humans and beastmen have a very rocky relationship but this is a major peace talk where every word and interaction matters but this man actively tries to sabotage the whole thing by blaming the beastmen. Honestly with how much the dad blames the beastmen (with no real evidence to be honest, like yeah leane is the strongest one who could have killed the guy in a fair fight but I don't really think the dad actually knows this) I wouldn't be suprised if he was the killer, if he wasn't such a fool and far to weak to make a sneak attack successfully.
If the father didn't exist I don't think the story would be too different honestly, he just causes more problems that aren't really needed as the story continues.