Have you ever read 'Odds End' by Tim Wynne-Jones. It is a very similar sounding plot. The person wanting to take the house from the occupants is quite subtle in some of the things he does. Trying to kill someone would sooner or later elicit interest from the police, so you'd have to be extremely careful to do things that don't go too far. If those that want the house do kill the owner, the police will remember that he has made claims someone was trying, and bring an unwanted investigation upon them.

https://www.fantasticfiction.com/w/tim-wynne-jones/odds-end.htm

You could try hair gel smeared on the stairs, with a squeezed out tube lying just under a cupboard nearby, or a dead bird blocking a chimney for a gas fire (Carbon monoxide). Tablets swapped between bottles, so that multivitamins are swapped with sleeping pills. All of these would be difficult to prove as attempts at murder, and if the householder were alert to dangers - he would discover them before they hurt him, but they would rattle him.