White is not by nature prone to yellowing. Some paint are (the organic component of the paint), but if you are using a lime wash it will never turn yellow and TiO2 as a pigment is very stable.
What gets it dirty on the roof? You also generally can't see the roof except looking down from taller buildings?
Houses aren't cube shaped, generally. The roofs are sloped and the front side can easily be seen from ground level.
In some places, yeah. In my experience buildings higher than about 5 stories do have flat roofs ("cube shaped"). Almost all housing in my hometown don't have sloped roofs because it's mostly apartment buildings rather than detached homes.