>Similarly, with building corners under trees I sometimes can't decide whether the building is rectangular with a corner shaded out or has a more complex floorplan

You can use the "MML Topographic Map" layer, one of the default layers in iD to trace buildings, so you don't need to guesswork the shapes. Some of the very complex shapes they don't represent well in that data, but for typical buildings I find it quite accurate.

>Properties fully surrounded with fences: I don't have local knowledge, but I assume most of the cases it's not a big gap in the fence but a small gate

Typically in finland (I have not visited the neighbourhoods in Äänekoski), but normally you'll find bush/hedges as fences between properties, sometimes wooden fence and sometimes no fence. Other materials, like masonry also do exist but not very common.

Gates are quite rare. I would say that out of 100 buildings maybe 1-2 would have a gate in the driveway. Also typically houses like these have car-driveable driveway, so that gives you an idea of the minimum gap width too.