A home that has AC running half of the day will use a lot less energy in total than one running all day long. There are substantial efficiency losses from heat leaking in, which will be exacerbated by trying to maintain a larger temperature delta 24 hours of each day. A house is not a perfect temperature battery on the scale of hours, far from it.
Depends on house type. For american-style wooden houses that might be true, but where buildings are built primarily using concrete with outer insulation, their thermal mass and inertia is much bigger.
