You're right, GPS accuracy is not always good. However here we are talking about aerial imagery. Those can have an offset, but they have the correct scale (at least in this case). 

What this means is that the building may be shifted a few meters, but it is the correct size.

Regarding merging those two parts : you added a different number of "roof levels" (1 and 0) on those two parts, that's why I haven't merged them yet. If you tell me what's the correct value I can merge them :)

On other cases, as you noticed, usually balcony and related features are split, with the tag wall=no. This distinction is made in the French Cadastre, so it was mapped this way, even if on most buildings it doesn't bring a ton of value.

I would not add informations on those small dependencies (I don't think it's worth the time), but you can as long as it fits the reality. It depends on what you think is important to be in OSM ;)

