If the demon in question can persuade or change reality somewhat, it may be alright to have him dupe (as you say) Kurama for a while while they're in the Human World. If the demon is assumed dead from long ago, it's safe to say that he wouldn't be on Kurama's radar. Or at least an unlikely reason for such odd events happening. So you're thinking the power would need to be maybe transferred to the target? If Yoko thought he killed him, it would be safe to say the demon's power isn't where it used to be so maybe that is why the demon has to make direct contact with his future pawns/victims.

I think we'll need to have the suicides just start when Botan shows us. Since she is one of many reapers who know when people will die and such, I think two odd suicides that weren't predicted at all or make sense, would make Koenma want to know more. I do think there still will be a few more if the demon sees how much it is tormenting or troubling Kurama. I think if we have Botan and Kurama investigate in the beginning, it would make her less inclined to leave it when things get considerably more tough and dangerous.