1) There are official geodesic points all over the Province of Quebec, precise to the cm. This data has been used to calculate the shift of Bing Maps in the Montreal region, which is indeed 0.6,-1.54. See https://geodesie.portailcartographique.gouv.qc.ca/ for the complete list. Any precise alignment in osm should be discussed before doing it if the person editing does not have access to official positioning data. All our positioning is based on the official geodesic data and should not be moved. Also, there is no such thing as distinct alignment of buildings and alignment of ground objects. Buildings must always be aligned on the ground, so everything is aligned on the same ground base.
2) I do not care how parkings are mapped, as separate polygons or one huge polygon, because most of the time, we add the capacity to it, but at least it must be connected to a street or service road otherwise routing engines cannot know where is the entry point of the parking. Having multiple connection is fine, but having none is problematic.

Thanks for your understanding and thanks for editing OSM!