I'm not surprised that this happened in Shirogane. I remember the first time I went there before you could even buy the plots myself and my fc mates noticed a lot of them are quite cramped together. We anticipated there could be problems with neighbouring gardens having very tall furnishings obstructing the view of the fc plot, as well as tall roofs from housing skins. We ended up not moving to Shirogane for a couple of reasons, one of which was it seemed that for scenic reasons the amount of well placed plots is, in our opinion, quite small compared to the other housing districts.

I am curious about these trees that are causing lag issues for some of OP's friends. How many trees are there? It must be a crazy amount in a small space to do that.

I don't think the furnishing placement in this case can count as harassment. It sounds like poor design on SE's part is the real culprit. If the plots weren't placed in the manner that they are OP's problem may not exist.

OP, you and your fc may want to consider moving to another plot location. There are a lot of plots in the game that are secluded so your view you won't be greatly obstructed by the furnishing choices of other players. You could also try to find a plot that is on the edge of the map to reduce the amount of houses surrounding it.